So in the Sketch, it’s actually Maedhros who rescues Elrond (Elros didn’t exist yet) and not Maglor.
“Their son Elrond who is part mortal and part elven, a child, was saved however by Maidros.”
(The Fall of Gondolin, p. 243; “The Conclusion of the Sketch of Mythology”)
And in the Quenta, you can literally see when Tolkien changes his mind and switches around Maglor and Maedhros.
“But great was the sorrow of Eärendel and Elwing for the ruin of the havens of Sirion, and the captivity of their son, for whom they feared death, and yet it was not so. For Maidros took pity on Elrond, and he cherished him, and love grew after between them, as little might be thought; but Maidros’ heart was sick and weary with the burden of the dreadful oath.
[This passage was rewritten thus:
But great was the sorrow of Eärendel and Elwing for the ruin of the havens of Sirion, and the captivity of their sons; and they feared that they would be slain. But it was not so. For Maglor took pity on Elros and Elrond, and he cherished them, and love grew after between them, as little might be thought; but Maglor’s heart was sick and weary, &c.]”
(The Fall of Gondolin, p. 252, “The Conclusion of the Quenta Noldorinwa”)
Likewise, on the next page (255), it is first Maidros/Maedhros who says unto Maglor: ‘If that be the Silamril that riseth by some power divine out of the sea into which we saw it fall, then let us be glad, that its glory is seen now by many,’ but in the revision the brothers switch places so that Maglor is the hopeful one.
I’m so interested by this decision. Maybe it’s connected to making Maglor more sympathetic and the survivor, as Tolkien starts to realize that Maedhros is going to commit suicide and Maglor is going to wander the shoreline for all eternity. You can also see Elros take shape – as Tolkien starts to envision a future for this world, a Second Age, he invents the Father of Númenor and starts arrange things for the coming years, and that means killing Maedhros and making Maglor the more tragically sympathetic character.
Ever since I wrote this fic, I’ve had the headcanon that Maglor is more open with affection and has a better relationship with the twins, but that Maedhros loves them just as much if in a more subtle way, and this only affirms that idea. Maglor may have been the one to take initiative, but Maedhros is there to support them, too. I also really like the idea that Maedhros will make himself into the bad guy, especially as the twins are growing up, so that they will project their resentment and anger onto him and thus spare Maglor their wrath. Maybe that’s why the stories tell of Maglor fostering the boys and not Maedhros, though I can’t imagine that Elrond or Elros failed to realize what Maedhros was doing when they grew older and became fathers themselves.