I think I figured out what I thought threw a slight wrench into being certain that Tolkien's last idea was that Maedhros, not Maglor, took care of Elrond and Elros. First, a hopefully accurate characterization of the texts for the Tale of Years:
Text A little more than a copy of Annals of Beleriand, thus a pre-Lord of the Rings version, although there were some differences compared to AB 2 or the original TY that went with it. This version tells the reader nothing of note with respect to the Maidros or Maglor question ('529 The Third And Last Kinslaying')
Text B includes corrections, interpolations and alterations made to A -- but this text shows no significant alteration to A with respect to the Maidros or Maglor question, thus it is still vague on this point.
Text C new version, and here Elros and Elrond are fostered with care by Maidros, nothing about who forswears the Oath however.
Text D typescript (termed D1 where it is type, and D2 when it reverts back to manuscript) In the manuscript part of D (thus D2) Maidros forswears his oath, but D does not go far enough to say who fostered Elrond and Elros with care.
Christopher Tolkien comments: 'That Maidros 'forswore his oath' was stated in AB 2 (V. 142); in this and the following entries my father was following that text very closely (indeed D2 is based upon it throughout)' And in his last comments CJRT remarks that D2 might derive from JRRT's work on the Narn: 'But this is very uncertain; and if it is so, it is the more remarkable that he should have based these entries so closely on the old pre-Lord of the Rings annals.'
So this made me question: what if Tolkien was merely echoing AB 2, a pre-Lord of the Rings version, without realizing what he had written in Quenta Silmarillion, which, although the mid to later 1930s version skips over the Third Kinslaying, it is Maglor who desires to submit to Fionwe (Eonwe), but he yielded to the will of Maidros as to how they should lay hands on the Silmarils.
At the time 'connecting things' I took this to possibly indicate that it 'should' also be Maglor that nurtured Elrond -- that is, if Tolkien was simply echoing AB2 concerning Maidros -- but now I notice that while Maidros does forswear the oath as in AB2 -- it was still Maglor in AB2 that nurtured Elrond. So it's arguably difficult to sell the idea that Tolkien was just following AB2 here without thinking of Maglor's reaction to Fionwe in QS (and if QS is later than AB2).
Sketch Elrond saved by Maidros :: Maidros and Maglor submit to the will of Fionwe, but Maglor steals a Silmaril
Qenta Noldorinwa I Maidros takes pity on Elrond and nurtured him :: Maidros is minded to submit to the will of Fionwe
Qenta Noldorinwa II Maidros cherished Elrond > altered to Maglor taking pity on Elros and Elrond :: Maglor is minded to submit to the will of Fionwe
Early AB Maidros forswears oath :: Elrond nutured by Maglor :: no text on reaction to Fionwe's will
Later AB Maidros learns of the uprising of Sirion's Haven and forswears his oath :: Damrod and Diriel ravage sirion, Maidros and Maglor are there but were 'sick at heart' :: Elrond was taken to nuture by Maglor :: Maidros and Maglor driven by their oath seize the Silmarils that had been taken by Fionwe (no mention of one brother yielding to the other here, but we are in a relatively brief tradition here in any case)
Quenta Silmarillion mid to later 1930s [gap includes Turin's outlawry to voyage of Earendil] Maglor desires to submit to the will of Fionwe but yields to the will of Maidros :: Elrond noted as being with Maglor -- although this is noted after Maidros had perished anyway.
Tale of Years (as above)
So looking at things in more detail now, I can see that in AB2 I really don't have the same person who :: forswears his oath :: also takes care of Elrond (or Elrond and Elros) :: and also is at least more willing to submit to the will of Eonwe. If D2 was simply based on AB2, then one might assume that Maglor took care of Elrond and Elros as well, but D2 doesn't get far enough, and arguably text C would imply that this wasn't going to be so.
And as Lord of the Rings correctly notes, Tolkien doesn't get far enough in The Tale Years to even suggest who yields to whom concerning Fionwe; and if we go back to Qenta Noldorinwa I, we see that it was Maidros who was more willing to submit to Fionwe.
It makes sense to me to have the same character forswear the oath, nurture Elrond and Elros, and be more inclined to submit to Eonwe. The only thing I would add, however, is that Maedhros was the Eldest, and one might think his will would sway Maglor, although that's not necessarily true of course, just based on that.
By the way, none of the [again, hopefully correct] textual characterizations above were posted because Lord of the Rings had described anything incorrectly, I just wanted to lay it out in a different way.