Justin....First, I wanted to say that that was a really nice post and you raised some good points. But I want to clarify that (perhaps contrary to what you would call "orthodox" trinitarian approaches), I was not construing "firstborn" in a strictly metaphorical way. Jesus is "firstborn" because he is the first to be raised from the dead and made a "son" of God, and as the firstborn he has inherited all things as his birthright. If he was "appointed" to be firstborn (cf. Psalm 89:27, "I will also appoint him my firstborn, most exalted of the kings of the earth"), and even if his inheritance due to his being appointed (cf. Hebrews 1:2 "the Son that he has appointed to inherit all things"), he is still the firstborn he was appointed to be. Moreover, I think Paul may have indeed construed the resurrection as a begetting: he describes Jesus' resurrection in terms of God giving his Spirit and power to bring life to the lifeless Jesus, so that Jesus was "declared to be Son of God ... through his resurrection from the dead" (Romans 1:4), just as anyone who is born receives life from God; in the case of Christians who are adopted as sons, who receive everlasting life through the Son, Paul refers to them as "born by the power of the Spirit" (Galatians 4:29).
So, at least in my speculative exegesis, Paul was using "firstborn" in its usual sense of the term....Jesus was the first member of a much larger group to be born through the resurrection, and he has birthright rights by virtue of being firstborn (which he shares with his brothers, cf. Romans 8:20-30).
Certainly, in Colossians, "firstborn of all creation" is parallel with "firstborn of the dead." Jesus was the first to be born from among the dead (in an eschatological sense), and he was first to be born in creation.
This is where I disagree....the two expressions are NOT in fact equivalent: prótotokos pasés ktiseós "firstborn of/over all creation" in v. 15, and prótotokos ek tón nekrón "firstborn out of the dead" in v. 18. The first expression has a genitive relation, which could either be a partitive or a genitive of subordination, whereas the second has the preposition ek "from" which indicates that the "firstborn" was once a member of the group of "dead (ones)". If we assume that the genitive is partitive in v. 15, then we would have to assume that prótotokos means two different things: (1) "firstborn" as the first created being, and (2) "firstborn" as the first resurrected being. My interpretation is that prótotokos means the same thing in both contexts (#2), and thus it has this sense in v. 15 (i.e. "the firstborn [from the dead] over all creation"). In my first post in this thread, I also gave two exegetical reasons from the text itself that invalidates a partitive understanding of the genitive case: (1) the word for "all" would shift its meaning at least three times in the text, variously including or excluding Jesus, and (2) the logical relation expressed by hoti "because" in v. 16 makes sense if the genitive conveys the concept of Christ as supreme over "all creation". In support of the genitive of subordination interpretation also is the whole point of v. 16-18 which states over and again all the different ways in which Christ is supreme over all things. So I do not view the interpretation of prótotokos pasés ktiseós as "the first to be born in creation" as best befitting the context, imho.
I also recognize that there were other early concepts of Jesus' begetting, including the quasi-Platonic concept of the begetting of Jesus as God's first thought or logos, the adoptionist concept of begetting at Jesus' baptism (cf. in Ebionism), the creation/begetting of Wisdom in Proverbs, etc., but I did not discuss these (as well as the valuable apologist distinction between begetting and creating) because I do not believe that these are representive of Paul's views; Paul's gospel is almost entirely focused on Jesus' death and resurrection and discusses Jesus' Sonship in those terms (Romans 1:4).