The other thing I thought was odd was that Umbridge blasts through the door of the room of requirement. ..........
If you know the purpose of the room, you can enter. Remember Cho’s friend told them the purpose of the room, so she was able to come in. Of course, like you pointed out though if the audience didn’t read the book, then they won’t know this info and will be confused.
I was a little puzzled why Cho was the sneak. I know it was easier then introducing a new character and it explains why Harry doesn’t want to hang with Cho anymore, but the way it happens they sort of make us have a little sympathy for her. Umbridge threatened her moms job and apparently used the veritaserum (sp?) on her to get her to talk.