So there’s a note in the Silm saying the Noldolantë was made ‘ere Maglor was lost’ which is why they know about it. So I think it’s safe to say he wrote and finished it before that final silmaril run.
I’m thinking he wrote it and before Elrond and Elros went to Gil Galad, they took a copy each. Maybe Maglor wrote it at their request. Maybe he sent it with them to make sure history wasn’t forgotten. Maybe they wrote it down as he sang in the empty rooms his little brothers once slept.
I think Elrond kept his copy close to his heart, hidden away for a time it would be safe to reveal. I think he would insert verses here and there in his own songs, until the entire thing was split over a hundred different pieces. I think he didn’t reveal it until he got to Rivendell, where he had several copies made.
I think Elros made his copy public, and it was well known across Numenor for many a year, until like many other things, it was lost in the Drowning. Possibly a little before when they wanted to distance themselves from the elves. I think it didn’t come back to Gondor and Arnor until Aragorn became king, a song he’d heard since childhood carefully transcribed in Elrond’s own hand.
With three verses added at the end.
One for Maedhros, lost to the fire.
One for Maglor, lost at sea.
And a final verse for Celebrimbor, caught in the Doom despite his kindness.
No wonder the Noldolantë didn’t make it into Pengolodh’s writings.