Or is it once it ends it ends? I kinda want to help all those lost souls move on but i'd rather focus on the main story first.

If there is a hard ending does it warn you before you enter the final investigation area?

No, hard ending. It technically does warn you but it's a false warning, because once you're done with the Museum, you can't go back. Stuff everywhere else can be revisited later though, so if you're really worried about missing stuff, maybe use a collectables guide for the Museum area.

Sucks that there's no in-game map, guess i'll have to search for some user created ones.