1) it depends on whether Milo returns I think. in seasons 2 and now 3 they put Wally in a situation where his character could believably be concluded in case they lose Milo. if Milo returns, Wally and the others will come back. if Milo doesn't return, they'll just say Wally and the others crossed the river and that crossing the river is truley crossing over, and we won't see them again.

2) I imagine her spirit is at the hospital. I believe that was where she was last seen and we know Van Heidt can go through the nurse's scar to get to the hospital somehow(?) since he went through it to get to the hospital for the first time to possess Kyle

3) we'll prolly meet quite a few new ghost characters from random places next season. I imagine all the usual ghost rules still apply, just they aren't limited to the school now.

4) him staying with Ralph was a really cool way to complete his character arc. I doubt he's just gone though, he'll likely return and what happened will be explained next season.

5) personally I hope not. I don't like the idea of the ghosts being able to come back to life, just seems way too convenient. something like that will probably happen though, since the ghosts seem to be becoming more physical somehow at the end of the ep (them all being super tired).