Off to the visitation (turns out the service will be at some other time, possibly very limited and private–unknown if I’m invited yet.)
But I thought I’d post up the first two parts of my reply.
I am very grateful to have learned new material, which I will be incorporating into a revision of the document. (Although there are also ‘corrections’ which are not really ‘corrections’, on closer examination. But we’ll get to those along the way. 
) The OT section is less lengthy than the NT section, partly because the spread of data (or the disputable data anyway) is just less dense by proportional weight to the length of the OT texts overall, but also partly for a reason I will discuss next as the first part of my reply.
I. The Goal of the OT Section
I’ll have something to say about presuppositional constraints of interpretation in the next part of my reply. But here I want to clarify that my goal for the OT section was not to argue that the OT texts teach a developed trinitarian doctrine–something I have never claimed to be true about the OT. Mainly what I wanted to do was provide an overview of the kinds of claims, concerning God and God’s operations, which would later help explain why NT authors (and Jesus by report) make the kinds of claims they do. Obviously this isn’t something that can be fully illustrated without going on to the NT testimony–where I will be referring back to the OT pretty regularly (sometimes in regard to examples mentioned in my OT section, and sometimes in regard to other OT texts). But this is the other main reason why my report on OT testimony isn’t longer and more detailed than it is. It’s also, by the way, why I don’t talk any more about the concept of representative agency than I do–because I will be discussing the notion far more in-depthly when I get around to Jesus and how He is treated (and treats Himself) in the NT.
(I also hold back from discussing representative agency much in the OT section, because I thought that the relevant problems associated with trying to claim something less than God’s own identity for a particular agent of God, were pretty well illustrated already in the examples as they stood. Which, considering that you often have to appeal back to a metaphysical constraint for interpretation, I’d say I was at least somewhat successful at that goal! 
But much more could be said about those issues and problems. And much more will be said, later in my reply. )
Was I at least trying to establish that the OT teaches doctrines that, when put together, amount to orthodox trinitarianism (minus specific incarnational issues)? Yes, but not necessarily to the exclusion of other possibilities–by which I mean that I wasn’t aiming for an absolute conclusion that couldn’t be greatly improved and clarified by considering the NT texts (and especially how they themselves reference and make use of the OT). If the presentation broadly illustrates how later readers (like in the days of the NT) could be inspired to interpret the data along such-n-such doctrinal lines, studying the scriptures in the hindsight of their experiences of Jesus and His teaching and deeds, then my goal for that part of the paper would be achieved.
It could be replied that, in presenting the material this way, I am at least arranging it (if not reading directly into it) according to how I already believe to be the teaching and even the intentions of the NT authors (and Jesus by report); when I ought to be interpreting the NT by means of the OT instead. Actually, I think I go both ways on that–for example, whenever the NT authors refer back to OT scriptures and apply them to the topic of God and/or Jesus, they couldn’t be being faithful to the OT testimony if they weren’t thinking in terms of OT contexts; consequently, if I want to get a clearer idea of their own meanings, I had better keep track of those OT contexts, too! But at the same time, the question has to be put: do the NT authors not have anything new, or at least newly clarified, to say about the meaning of the OT scriptures?! Unless, on some prior ground (or raw assertion?) we expect that answer to be ‘no’ (and I know of no good reason why we ought to expect ‘no’ beforehand), then it would also be a good idea to watch to see if the NT authors (and Jesus, by report) are trying to expand and/or clarify a ‘proper’ understanding of OT testimony about God and His operations.
This principle doesn’t necessarily mean that any NT author (or even Jesus by report!) is correct to be doing so; a non-Christian Jew might (and in fact almost certainly would) point to such-n-such in the NT and say “But they are innovating some drastically wrong theological idea here!” But then again, if it seems clear enough that Jesus and/or NT authors are teaching some idea that wasn’t clearly believed (or even believed at all yet) by the OT authors, it might be of some use to know if the OT texts could be read to mean that–even if the OT authors themselves did not understand the implications of what they were reporting. (If the NT authors, and/or Jesus, are teaching bodily resurrection for example, this is something that not all the OT authors clearly believed in; but which may have been foreshadowed by various things they did talk about and believe.)
This leads to the question of whether there could be a real and important testimony from the OT, the full (or fuller) meaning of which was not evident to at least the authors (if not the readers) of those texts. It may be that to infer, or more vaguely “to see veiled signs” of, a singular Trinity of divine persons “is to go beyond the words and intent of the sacred writers”. This would not mean such testimony wasn’t there, however. John the Evangelist understands a statement of Caiaphas to be a legitimate prophecy, granted due to the man’s appointment as high priest for that year, but still invites the reader to go beyond the word and intent of Caiaphas. (John 11:49-52) Matthew (or whoever wrote and/or finally compiled GosMatt) goes rather far beyond the evident meanings of Jeremiah in explaining Herod’s slaughter as being a fulfilled prophecy (Matt 2:18; Jer 31:15), even when the reader happens to notice that that portion of Jeremiah ends with a peculiar riddle (not directly mentioned in GosMatt) which could point to the forthcoming virgin birth (31:32b, “For YHWH has created a new thing in the earth: a woman will encompass a man!”) For that matter, Matthew goes far beyond the evident meanings of Isaiah, too, when explaining that the virgin birth of Christ by Mary fulfills the prophecy about “Immanuel” (originally a child born to the virgin wife of the prophet himself, Is 7:1-16; midrashed in Matt 1:23.) Yet most unitarians acknowledge, just like most (nominally all) trinitarians, that the virgin birth of Christ is being testified to in GosMatt and GosLuke.
I don’t mean that this gives anyone an open field for going beyond the evident intention of the OT authors. I am only illustrating that it is not, in principle, impossible that what prophets say and even mean (whether loyal like Isaiah and Jeremiah, or rebel prophets like Caiaphas!), may have larger meanings than they themselves are aware of at the time; nor is it impossible that they themselves may have some distant inkling of them, even if “the secret hushed in eonian times” is manifested and heralded later besides through prophetic scriptures. (Rom 16:25-26)
And, from a wider critical standpoint (including the possibility of a sceptical rejection of OT and/or NT testimony on the relevant topics), it might be discovered that the Judeo-Christian canonical scriptures are talking about something that ought to be rejected as a metaphysical falsehood (or even an outright contradictory impossibility). This leads to the topic of presuppositional constraints in interpretation; and so on to the next topical header of my reply.
) in order to get some skin things worked on (moles removed, etc.)
But I like to revise and update it occasionally as a self-critical exercise, which is one of several reasons why I’ve been posting it up in pieces on this forum for about a year now.)