Not trying to talk you out of it Warren, but…
3.) (out of order obviously, but I wanted to state an important agreement first) Fundamental reality is what it is, and we can either cooperate with it or not, and that will still be true even if fundamental reality fails or stops or never even intends on saving sinners from sin.
Keeping in mind I’m someone who thinks any theism less than ortho-trin is ultimately incoherent, and that any soteriology less than universal salvation is also ultimately incoherent (and typically either accidentally or intentionally non-trinitarian at bottom, so incoherent that way, too 
): I do agree if God turns out to be non-universalistic after all we’ll just have to make do with it the best we can or else be in knowing rebellion against the source of our existence.
(This agreement includes (4) and (5) in principle and in practice, of course. 
)
Now, if you were really putting the exegetical issue aside for now you’d be agnostic on the topic instead of at peace for now with anni. But I’m saying that to assume in your favor that you regard the exegetical case as strong enough proportionately in favor of some version of anni vs ECT or Kath (in any variations), to make peace with believing anni (for now anyway). And that’s okay.
1.) I wouldn’t want to be the person trying to make an exegetical argument against God acting to cause some kind of inconvenient personal state to at least some sinners post-mortem; and the exegetical evidence sure looks to me like the personal inconvenience is greater than a painless euthanasia. (I’d add that I find the exegetical argument for any kind of hopeless punishment from the Romans “wages of sin” statement to be super-faulty, but that’s beside the point.)
From a purely philosophical standpoint, though – since Glenn is first and foremost a philosopher – we aren’t even talking about (mere!) supernaturalistic theism yet if God isn’t actively keeping people in existence; consequently (assuming annihilation for purposes of argument) people could only cease to exist as such due to God’s active choice to stop maintaining them as persons in existence. Yet God doesn’t simply annihilate any sinner out of existence immediately upon their rebellion against the source of their existence. So obviously there are other factors about God’s intentions for the person.
In other words, in at least some (and apparently all) cases, God adds something to the situation other than straight-forward death.
But on Glenn’s position (as reported) any suffering God adds beyond choosing for sinners to cease to exist would be unnecessary and not particularly loving; which would necessarily have to include raising the wicked (bodily or not, but especially bodily) to the judgment of annihilation, and also any disciplinary action by God before death. Just how far is Glenn prepared to go in denying generally accepted actions of God, in order to be logically consistent on this point?
2.) Does an 8% annihilation (or any small number) really mean God is a total failure? Depending on what God’s goals are, yes, it does. For example if God pledges His own name and self-existence on the success of X, and only partially succeeds at X with no future success possible, that final failure would be catastrophically total for all reality. Or similarly, if the Father and the Son (God self-begetting and God self-begotten) covenant with one another to accomplish X but one or both partially fail at accomplishing X, that’s ontologically catastrophic.
Similarly again, if God’s own active self-existence upon which all reality depends for existence, involves God fulfilling fair-togetherness between persons, then any action of God toward ultimately fulfilling non-fair-togetherness between persons would be for God to act against His own source of (self-)existence, just as if we did that, even (just like us) if the person so acted against was only a creature. For God to be finally defeated along this line (instead of choosing to act such a way) would still be equivalent to defeating God’s own self-existence.
Relatedly, though perhaps less catastrophic, if the goal of the Son’s judgment in raising those who do the bad things to “eonian judgment” is to bring all persons to honor the Father (without even counting trinitarian or non-trinitarian theologies) as the Son honors the Father, and the Son fails to do so in even one person’s case, then at the very least this would mean the judgment entrusted to the Son by the Father has failed into a final injustice (either triumphing finally over justice or enacted by ultimate justice Himself). The higher the Christology, the more catastrophic this failure would be, but it’s nothing for even a non-trinitarian to take lightly as a proposition.
Obviously if God’s goal is to annihilate 8% of rebels and save 92% of them, and God has no higher goals conflicting with that result, then God would actually totally succeed by saving 92%! – and that’s a position a Calvinist would try to take, on what amounts tacitly or explicitly to double-predestination. (I think I recall Glenn being some kind of Calv, and this would be relevant to his own categorical soteriology if so.) But then there’s no reason to be talking about whether 8% annihilated counts as an acceptable failure rate or not: that can only be a broadly Arminianistic concern.