Right, quite a bit to say, got a lot of catching up to do because I can't post from my PC at home, grrr!
I think Cho would still have another year left at Hogwarts, I'm sure it said she was a forth year in book 3. I think all the names that were being put forward for the part of Cho, like Lori from Neighbours (whose from New Zealand btw) would have been too old, none of them looked 15.
Couple of other things I wanted to bring up while I was reading book 5 over the weekend:
How did Fudge get his Oder of Merlin 1st Class? You seem to have to do something to earn one, I'm assuming Dumbledore got his for defeating Grindewald, and Pettigrew was going to get one for trying to stop Black, but what has Fudge done to deserve one?
Another thing, what NEWT's will Harry take? McGonagall tells him he needs 5 to become an Auror, but then only mentions Charms, Transfiguration, Potions and Defence Against the Dark Arts. The only other things I can think of are Herbology or Care of Magical Creatures.
Final point, what would have happened if Neville had removed the Prophecy from the shelf in the Department of Mysteries? Would he have gone crazy? And does Harry being able to take it confirm once and for all that the prophecy was referring to him?