Why is Katie Bell still at Hogwarts in DH2?
I've been wondering the same thing. Both her and Cho Chang are a year ahead of Harry. At first I thought that in the scene where Snape is talking to all students in the Great Hall, they were wearing school robes just to appear as students and blend with the rest, but then you can clearly see that Snape looks at Cho and nothing happens. So I think it was a movie mistake.

IMDB says Sean Biggerstaff is in the movie as Oliver Wood. Did anyone actually see him?
He was there for about 3 seconds. At least that's all I got. He is the one commanding the attack on the giants. You can see him on his broomstick ahead of a group people yelling, "Come on!" or something like that. I really wanted to see more of him.

I won't swear to it, as I couldn't exactly see clearly, but did George's ear grow back? What's up with that?
Well, in the scene where George asks Fred, "Are you all right, Freddie?" I see no ear when he turns his head. Here, take a look: link

As to the last question, he totally does! :D

And I was wondering, whose body was Professor Trelawney covering up in the Great Hall? I didn't catch that and I want to know.

Thanks for the info, twifanpire. =)

If you're talking about in The Aftermath when Trelawney's sitting there with a Patil, I think that was Lavender they covered up.