YES! We need a whole Young Avengers/Runaways crossover focused on finding out what the heck happened to Xavin.
Well, I think Young Avengers has more potential to go into the Young Adult territory, since it's a franchise with actual young adults (I mean, Amadeus Cho is supposed to be a young adult as well, but it looks like Marvel doesn't remember that). But even if the marketing/demographic is similar, I don't think it's any problem. It's not like we don't have multiple other books with the same marketing/demographic being published simultaneously as well.
I don't see how. Unlike many previous teen/young groups where most of the characters have vanished, a lot of the characters from YA still managed to stay relevant recently: Hawkeye, Ms. America, Wiccan, Hulkling, Cassie, Noh-Varr... it shouldn't be that hard to make a reunion happen. We just need a writer and editorial willing to.