A very possible scenario, although Sauron getting to Gil Galad would be quite the departure from Canon. Also the Elven three are supposedly the ones Sauron had no hand in creating, so there is that. But doubtless the Elven decay will play into the forging of the Elven Rings.
"An off screen Annatar advising Celebrimbor" was one of my top Sauron theories already, and remains there. Halbrand looks less likely now, the Stranger is back in the race (though frost and Sauron are seldom associated in the books, but the only people in ME with notable cold magic links are Morgoth and the Witch King, both of whom are highly improbable for obvious reasons - and Sauron was previously traced to Forodwaith by Galadriel), Adar is still very possible (him flipping out over the name means nothing; Aragorn notes in LOTR Sauron does not permit his minions to speak his name), but perhaps too obvious.
Very meh feelings about the new origin of Mithril, mostly becsuse I cannot conceive why there was any need to change the fate of one of the three Silmaril for it, seeing as "a silmaril sank into the depths of the earth, its light reached into veins of silver and turned them to Mithril" would have worked *exactly just as well* and involved far less changes to canon. Possible the part about the battle will later turn out to have been elven legend.
If the elven legend about the battle does turn out true, then Adar seems a likely suspect to me for the Elven warrior who touched a Silmaril and fought a Balrog...and ended up not so pure after all.
(Or, time-traveling Gandalf, lol? I mean, the batt'e of Hithaeglir sounds a lot like the showdown between Gandalf - wand-elf - and the Balrog, ported back a couple ages early.)
Also, the clairification by Galadriel that Finrod was killed by servants of Sauron is nice in terms of aligning the previous broad outline more closely with canon.