The years and decades slide by. Mygwainor loves the time spent by her wife's side, loves the taste of - peace, of just being Luthien's wife so openly, of no shadow hanging over their heads. She makes friends. Taunts the Valar some, but... Not a lot, really, in the grand scheme of things. Feanor gets the printing press and typewriter perfected and in production fairly quickly, and the writing output of the Noldor explodes after a short lag. (Feanor is responsible directly or indirectly for a lot of that.) The elves relax around Mygwainor over time, and she settles more fully into their house than she has...
Anywhere, really.
She takes her wife on trips too, of course, travels Aman alongside her. Tells her stories about all the wondrous things she's seen over the course of the loops - she tells Luthien the strategic parts, obviously, the default arc of history and at what points it can bend, where does it break, but most of her stories are about things Luthien's never experienced. The time before the Awakening of the Elves. The millennia of history of the Years of the Sun. The rise and fall of empires, the tragedies and triumphs, the advances and discoveries. (It's pretty clear over time that Mygwainor fundamentally considers herself aligned with humanity, finds their stories the most interesting, but of course she knows plenty about the other peoples of Arda.)
She mentions here and there whenever someone familiar to Luthien was in a different role in any given story.
It becomes apparent over the course of these that she's dancing around some things. She's never spoken more than in passing about her zeroth life, of course, except that she was a mortal and that she hated the Melkor before her, but more than that...
Sauron is on the edges of a tremendous number of her stories in some capacity or another. It's clear that a different actor, a woman, filled that role before Ares.
It's clear, also, that a different actor filled Luthien's role until recently.
Mygwainor avoids talking directly about either of those facts. She speaks in distant tones about the usual script Luthien's life follows. Avoids discussing the Sauron before Ares directly, something sad and pained in her voice when she does come up.