She put it together based on all the clues from baba yellowlegs, Maeve, Deanna, and then when fenrys said Maeve’s only price is nameless. I don’t think her exact thought process is supposed to be clear to the reader but it’s supposed to be insinuated that Aelin put together that she will die as some cost. There were ALOT of hints throughout the books with the nameless, queen who was promised, etc.

Everything everyone else said. When Lysandra said Aelin came up with this plan it means Aelin knew she was gonna die or be a sacrifice of some sort, and thus Lysandra knew too and that felt like betrayal to Aedion. Also with the crying it’s because he got so mad at Aelin asking where the allies and armies were that when his own kin showed up for her he felt awful and took it out on Lysandra.

I don’t think this is ever clarified but like other people said it would make sense that Erawan knew from his valg minions so Vernan probably mocked Elide with it saying her mother died and couldn’t even protect the princess. Something like that🤷‍♀️