Why did lily go with him?

Didn’t

They are not allowed to take the kids without going through the proper procedure.

From what I’m gathering I think mori mom wrote a letter the “high council” that delver is needed in Lilys life cause they have healing abilities which is why she survived more than three weeks and isn’t able to eat or function without him. It seems they’re going to make an appeal(?) maybe?? I just know that it seems like they’re gonna try to communicate with each other and that means so much more to me

They're pretty much orphans, and just like orphans you'd need proper paperwork and approval before you can adopt one. Otherwise it'd be kidnapping. Miria already wrote a letter of recommendation and Delver seem to almost get the approval of Lily's custody

Everything has a process, and same thing might happen with Lily like how Mori experienced