The confusion began with your post 568.
https://forums.carm.org/threads/codex-sinaiticus-the-facts.12990/page-29#post-1026186
It was hard to quote that post and retain the formatting. The quote box loses the hot link and the quotes from Metzger et al end up out of the quote box.
Anyway, nothing real significant at this point, I may return to the heart of the matter on Song of Songs, the quotes you placed from Lost Keys by Jay Curry Treat, which got lost in the shuffle of Metzger et al. They were helpful but there is much more that shows that Sinaiticus lines up best with late Latin manuscripts.
"lines up best" - just weasle words, as the rubrics are by no means the same as in the Latin.
And that the best explanation is the simple Ockham-friendly one, that Sinaiticus was made with knowledge of that sophisticated formatting that is not anywhere in the Greek tradition except Sinaiticus. There is no good explanation of the bumbling scribes of Sinaiticus having a sophisticated view of Song of Songs, e.g. in a 3rd-century exemplar. Perhaps if Sinaiticus was produced awhile after Theodoret, whom you mention, the manuscript might begin to move in the direction of the advanced formatting. However, the best connections remain the late Latin manuscripts, and Jay Curry Treat is strong in explaining that Sinaiticus is with the Latin group, with some support from auxiliary languages.
Again and again the scholars have to struggle with their 4th-century presupposition, and give explanations that really do not work.
The only material facts are that the scholars accept that Sinaiticus is 4th century, and was by no means the first Greek manuscript to introduce rubrics, and was part of an evolving tradition.
p.421 "Any allegorical elements in the Greek rubrics are few or subtle in comparison to the
rubrics of Codex Amiatinus (Latin circa 700AD). It seems likely that the earliest rubrics in Song of Songs
would have been similar to the Alexandrinus rubrics - short indications of the speaker
placed at various points (but not exhaustively) through the text."
p.507 "The easiest way to account for almost all of the differences between the Greek and
Latin rubrics is to say that the differences mark those places where new developments have
been added to the text in question. For example, in the Latin rubric at 1: 12 (vox sponsae ad
sponsum, "The voice of the bride to the groom"), the word vox, "voice" stands out as
being untypical of Sinaiticus-tradition rubrics, but quite typical of Amiatinus-type rubrics."
"The Sinaiticus tradition of rubrics was not a static tradition. Nor was it a pristine set
of rubrics that was simply corrupted and adulterated with the passing of time. It was an
evolving tradition rather than an "authored" work.
It was a living tradition that developed
and reflected the various needs of its readers over the span of at least a millennium. Like
most traditions, it was, to borrow a phrase from Song of Songs 4: 15, "a well of water,
alive and coming down in a rush from Lebanon."
This is similar to the attempts to tie Sinaiticus to the Andreas commentary in Revelation. The truth is simple, that the Andreas commentary was available to those who made Sinaiticus. The backwards thinking tries to make Sinaiticus some type of precursor to Andreas, which simply makes no sense.
Beyond that, there are specifics of manuscripts that line up with Sinaiticus scribes and correctors. This is quite unusual and surprising.
The first thing to consider if a manuscript lines up with Sinaiticus corrector is ... was the manuscript used for the correction? However the scholars are not allowed to think in this manner, since it goes against the sacred faith of the 4th-century Sinaiticus.
Gaslighting.
