Not sure if your question was ever answered; here are my thoughts on that...
Regarding your "why Mid-Acts"* and why isn't it the whole NT and OT," it is not that it is not the whole NT and OT, rather; where the whole NT and OT are being viewed from.
From a Mid-Acts* Perspective.
This, the result of Induction - the gathering of facts towards their contrastive analysis in an attempt to arrive at an understanding of what they point to.
Other's "one gospel" conclusion differs due to how they apply Induction.
Both sides are looking at the same information, but how they are looking at it, differs.
Because each is actually applying Induction, differently.
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* Or, whatever the label Paul's distinctive ministry is referred to.
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As another example, say, someone holds to Partial Preterism, or Amillennialism, or some other perspective - that is just a label as a means of identifying where each is viewing all Scripture from, in contrast to where others are viewing all Scripture from, when describing differences in understanding.
The question is not the label, as all a label is, is a handy means of talking about things. As in "Put that 'container of beans' in the cupboard, and put 'that container of eggs,' in the refrigerator."
And at some point all parties involved in that exchange came to hold to those labels - container, beans, milk, cupboard, refrigerator - as basically communicating the same understanding between said parties.
Where the nit picking comes in is not so much in difference in labels, nor in difference as to what goes where - the beans the frig, the milk in the cupboard, for example - but in the need that some individuals feel compelled to - to nitpick.
Out of some personal issue such individuals then rationalize, into some personal mission as their means of justifying their actions against others out of this personal quirk of theirs.
I suspect these "O foolish Galatians," may once have known better. That they are simply too far gone in their need to lord their issue over others, "that they might glory in your flesh."
Rather than liberated by truth, such seize it and make it captive to their bondage with them. Never really freed by the truth they profess; such cannot but be compelled to go out and attempt to do likewise as to others.
Galatianism. Plain and simple...
"Their conscience" no longer "accusing" them, rather "excusing" their "need." Their conscience long since "past feeling."
But again, Mid-Acts is just a label summarizing a perspective from which all Scripture is viewed as it is approached.
Case in point, rather someone like Darby was, the first or not, to begin to see all Scripture from what came to be known as the Dispensational perspective; the real issue is whether or not his basis was sound.
And the meaning of "soundness" differs: is dependent upon; where each individual thinks he or she, is looking at things from, in contrast to where they might actually be looking at things from...
...as much as nit picking is based on what one feels compelled to have to pester, and pester, and pester another with, all the while feeling "right" in their action...all the while, an example of "bondage."
A neon sign on a dark, lonely road - "Danger: Bondage Ahead!"
