Well, talk about spoilers! Harry and Hermione would have one hell of a time deciding what was safe to change (aside from deciding whether to tell Albus or in fact anyone else).
I'm assuming that they can have more than one go at the room, although, I wonder if the stories change as Harry and Hermione change their timeline. That would be fun. Of course, after they change one thing, the books might end up as unreadable since JKR wouldn't "know" what happened... or would she? Lots of possibilities there. I'd vote for no character additions and keep it pretty close to canon characters.
Looking forward to a continuation of this and your other unfinished story.