My apologies for not keeping up with this thread as much as I wanted to. (Especially since I very much appreciate DC’s contributions and wish to support his-or-her activities on site! 
) Aside from ‘work’ work being a little more hectic than usual, I’ve been busy hopping up and down on Rob Bell’s book over in another thread, which DC might agree is something worth doing. 
(Probably for more reasons than I myself give!)
As I said previously, I am not a fan of the popular universalistic argument from disproportionality, which is what I understand DC to be aiming at here (a popular argument often deployed by scholars in their defenses, too, going back through Gregory of Nyssa and his day at least as far as Origen if not also a generation earlier to Clement of Alexandria whom I vaguely recall having something to say along this line as well.) Quantitative disproportionality of result is explicitly acknowledged in the scriptures, and universalists who have been poking around a while ought to be very familiar with at least one of the chief relevant examples since we ourselves appeal to it!–namely Romans 5, where although by one man all/many have fallen by one Man all/many shall be made just and restored.
Consequently I have been aiming my critiques in the direction of polishing and refining DC’s rebuttal argument against a universalistic argument from disproportionality per se. I could be critiquing it instead from the direction of the question of what constitutes justice and so whether a result or goal (disproportionate as it may be) may in that light be considered just or unjust; but that puts the focus on a different element of the argument (popularly represented by the universalist) and the rebuttal (of the popular argument).
I think this would be very much worth doing, but it would be topically different from what DC is aiming at, so perhaps it would be better for me (or him) to take it up in another thread. Be that as it may.
If the topic is agreed to be quantitative disproportionality, let us rephrase the [P]unishment proposal again more clearly. I suggest the following:
[P**] Necessarily, for any (S)inner, S does not deserve a punishment of endless duration for a sin of temporary duration.
It should be noted that this is not an argument but is, at best, a conclusion of an otherwise unstated argument. How the universalist is supposed to have gotten here might be important in assessing it (and perhaps rebutting it.)
For example, myself and many universalists (practically all dogmatic Christian universalists in my experience, whether or not they are ultra-u) would very quickly agree that so long as a person continues impenitently to sin, then that person deserves to be punished for that sin.
We would thus agree with (let us call it for now):
[P**a] Necessarily, for any (S)inner, S deserves a punishment of endless duration for a sin of endless duration.
We would also (obviously) agree with the related proposal:
[P**b] Necessarily, for any (S)inner, S deserves a punishment of temporary duration for a sin of temporary duration.
–which would be one of saying (perhaps not the best way) that if S repents of her sin, then S deserves to be punished for only a temporary duration.
[P**b] seems to imply as a corollary:
[P**c] Necessarily, for any (S)inner, S does not deserve a punishment of endless duration for a sin of temporary duration.
Which is the same as saying that if the sinner repents it would be unjust to continue punishing that repentant sinner endlessly.
And now we have a proposal identical in all elements to [P**].
The key principle of appeal here, although unstated, is not some kind of inherent injustice in disproportionate results per se (although universalists do sometimes tend to put it that way), but neither (despite the immediate form) is it really about quantitative parallels. The underlying notion is that the temporary sin is not the sin that happens in a temporary period of time and then is over with, but the sin that is repented of. A moment’s sin, unrepented of, may be still being held impenitently. (Or possibly it is deferred in a sort of Schroedinger uncertainty until the sinner is made aware of it again so that the choice of penitence or impenitence is now being made. “Today is the day of repentance”, for as long as it is called “Today”, but so also might it be the day of impenitence, too, for as long as it is called “Today”!)
In other words, the complaint of the universalist, though perhaps poorly put, may be that a person does not deserve an ever-continuing punishment for a sin of which the person is repentant. Which could also be said: it would be unjust to continue punishing a person when the person has justly repented of the injustice for which the person was being punished.
That is a rather different appeal in principle than to appeal to disproportionality as inherently unjust in itself, even though the wording of the complaint might be identical: [P**] Necessarily, for any (S)inner, S does not deserve a punishment of endless duration for a sin of temporary duration.
(Which is still aside from the question of what constitutes justice; which in turn could make a huge difference to the feasibility of the proposal or rebuttals against it, pro or con.)
At any rate, I still recommend revising to [P**] from your [P*], insofar as you acknowledge that “infinite here just means ‘endless duration’, not ‘actual infinity’.”
Now, if [P**] is acknowledged as more properly stated, how does that affect [R] or rather [R*]?
(Reminder) [P**] Necessarily, for any (S)inner, S does not deserve a punishment of endless duration for a sin of temporary duration.
Would it be?: [R**] Necessarily, for any (S)inner, S deserves a reward of endless duration for a sin of temporary duration?
That could hardly be true in the sense of anyone deserving a reward for doing a sin! But it might be true in the sense of anyone deserving a reward for repenting of their sin.
(This naturally introduces the question, if not already so, not only of what finally constitutes justice, but who exactly is it doing the rewarding and punishing and why. Are (S)innners saving themselves by their works independently of some authority empowering and/or judging them? Condemning themselves by their works independently of some authority empowering and/or judging them? Any dogmatic Christian universalist would most likely say no either way, and emphatically so. Theoretically I would expect any Arminianistic or Calvinistic Christian soteriologist to agree in rejecting either proposition. The fault of the universalistic defense may then lie in having accidentally rejected what they would otherwise accept; or the whole problem may be some category error misunderstanding, by the universalistic defender of the proportionality argument and/or the non-universalistic rebutter.)
[R**] as above can hardly apply to Jesus, Who was not a Sinner; but I realize that your labeling convention arrived at your own [R**] (Possibly, Jesus deserves “infinite” Rewards for “finite” actions) by a different method. (Namely, you replaced the unethically connotative “sin” of [P]unishment proposition set with the neutrally connotative “action” for your [R]eward proposition set; and then replaced (S)inner with Jesus Who is a ~(S).)
Possibly [R**] should mean instead: Necessarily, for any (S)inner, S does not deserve a reward of endless duration for a sin of temporary duration? (Which would keep the parallel element of undeservedness between [P**] and [R**].)
[D**] meanwhile would be revised: Necessarily, any (S)inner does not deserve X of endless duration for doing Y of only temporary duration. Which might be supposed (by the universalist proponent of the argument against disproportionality) to reflect [D**A]: Necessarily, any (A)ct-er (sinner or otherwise) does not deserve X of endless duration for doing Y of only temporary duration.
I was originally inclined to accept Adam as counterexample [1], to what amounts to [DA] if not to [DS] (since [1] proceeds on the assumption that Adam is not a sinner but acts justly). However, on further consideration I am less so.
First, we are still setting aside what it means for Adam to have deserved X. (Whereas if we are getting into the question of what justice fundamentally means and so of what desert involves, and so also who authoritatively rewards Adam with X for Y and why, the whole shape of the case may radically alter. I am not for example remotely prepared to agree that Adam could have earned eternal life of himself by keeping the law, regardless of whether we’re talking about temporary actions or not.)
But since the thrust of this line of thought has been setting this notion aside all the while (including presumably from the universalistic proponent of the argument against disproportion), I will let that slide.
Next though, spelling out [1] in a bit more detail in synch with the ** set of reformulations, leads to an evident problem:
[1**] Adam would have deserved life of endless duration had he only temporarily fulfilled the law in the garden.
But surely if Adam stops acting to fulfill the law, and becomes a sinner (thus only temporarily fulfilling the law), he does not still deserve a life of endless duration by virtue of what he did temporarily do, does he?! After all, storywise Adam keeps the law for some temporary period before ceasing to keep the law. And what happens?–does [1] not have itself an obvious counterexample defeater?! If so, then [1] (once rephrased in synch with ** above) does not currently count as an agreeable defeater to [D**A].
(It may be noticed, by the way, that the argument of quantitative disproportion, whatever its merits pro or con, as deployed or rebutted by universalists or non-universalists either one, has nothing primarily to do with the qualitative category of “eonian life”: even non-universalists agree that Christians who already have “eonian life” still die, and that their resurrection in itself is not “eonian life”; and will otherwise insist, unless they are annihilationists, that doers of injustice are raised to quantitatively unending life after having died, even though this is “eonian crisis”. But be that as it may.)
In regard to proposed counterexample [2]:
[2**] Jesus (vs. [D**A]) deserves endless duration of reward (e.g. glory, honor, titles) for the actions He undertook of only temporary duration in His active and passive obedience. (Relatedly, Jesus deserves endless duration of life for His people as His reward for those only temporary actions of His.)
My original complaint against [2] as a counterexample to [D] still stands per the sui-generis status of Jesus as God Most High as well as Man, although I see I did not explain this very well the first time.
What I meant was that, even in regard to the original [2] (Jesus endlessly deserves glory, honor, titles (reward) for the finite actions he undertook etc.) this is not a good counterexample as Jesus already deserved glory, honor and titles for being Who He is, the 2nd Person of the Trinity of God Most High. Any actions of finite duration (temporally speaking) He takes, either Incarnate or from on high (up to and including creating and sustaining ‘temporal existence’ at all!) does not materially add to this reckoning. If it is not really a question of whether He deserves X whether or not He does Y (and Calvinists would surely be the first people to strenuously affirm that He deserves X whether or not He does Y where Y involves saving any sinner!) then I do not see that [2] is a proper counterexample.
Moreover, it has been long acknowledged by many theologians across all three broad soteriology categories (Calv, Arm and KathRec) that the Son does only what He sees the Father doing and is the express revelation of the Father and indeed that what He does Incarnate (at least for revelatory purposes, but also in regard to fundamental properties of God) is only an extension to particular circumstances (small and close-up though that may be) of what the Son is always eternally doing (in communion with the Father and the Spirit) at the level of God’s own Self-Existence. Jesus deserves glory and honor and titles for Who He is and thus for what the Son eternally does (since fundamentally God’s own active Self-existence is Who He eternally is), and those actions are also expressed finitely (thanks to the on-going self-sacrifice of the Son in order to create not-God realities and entities at all) in His active and passive obedience. (Which by the way also includes granting as well as earning any eonian life for His people.)
This complaint of the sui-generis status of Christ carries over to a consideration of [2**]. The actions of God, even as the Incarnate Christ, are of any actions not of temporary duration, but are eternally real, because even the Incarnate Christ pre-Resurrection is not only or primarily a temporal entity. Our actions are only temporarily real, and only have any eternal reality in subordinate relationship to God; but the actions of God the Eternal have inherently eternal reality in relation to Himself.
But it may be said that this complaint goes too much into qualitative instead of quantitative disproportion. In which case I can answer by asking whether Jesus only temporarily earned eonian life (whether or not that is supposed to mean only quantitatively never-ending) for His people, or permanently earned it. If the latter, then [2] fails as an example; if the former, then what gospel is that supposed to be?–and wouldn’t a Calvinist of all people be the first to deny it?–and if we are not considering the eternal quality of the action or of Who Christ essentially is (and even Is!) then would Jesus in fact deserve reward of endless duration for only temporarily saving His people?? That seems dubious to me at best: if saving His people results in reward, then ceasing to save His people would seem to change the situation such as to cease the reward as well.
Admittedly I may be reading intention too much into the case there; but then I would have to ask whether intention is supposed to be irrelevant to the case. Are we talking about an impersonally mechanistic ‘deservedness’ or ‘undeservedness’? I would strongly deny there could be any such thing (especially as a trinitarian theist); and this may lead to the root problem in the whole matter, including for a universalist trying to appeal to injustice in disproportion: if there is no merely mechanical justice, including of merely mechanical proportion, then there is no injustice in merely mechanical disproportion of result to action either.
But neither should we (as universalists or non-universalists) be talking about merely mechanical ‘justice’ (or injustice) at all as though such a thing could even be true.
(Which at bottom would be my own rebuttal to any universalistic defense by appeal to mere disproportionality. 
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