Actually, the train is parked right now in Hogsmeade station, that's where the students unload, so that's probable where we would unload. And yes, J.K. Rowlings plans originally called for that when she was in talks with Disney, but they wanted to do the cheese thing and J.K. wouldn't allow it. The only problem I have with King's Cross being in Diagon Alley is that's not where it is in the books. But ah well, what are you going to do. Sounds like it would be a lot of fun, and they would be able to come up with more unique merchandise and get more money (which is all they care about remember xD)
--- Update ---
No it wouldn't be. Remember, we don't always have physical sets. They could very easily set up a Hogwarts express train simulator, perhaps one where dementors attack, and they could have the lady with the trolley selling pumkin pasteys and all of the other goods she sold...