We see through the books that there are numerous ways to get in and out of Hogwarts:
- By Train
- Flying car
- Secret passage to Hogsmeade
- Through the lake
- Through vanishing cabinet
It looks like there are a lot of way to get in and out of Hogwarts (except apparition).
What was the reason for such a complex plan (tweaking goblet, helping Harry to win, taking control over Harry's opponents)? It feels like it would be much simpler for fake Moody to just knock Harry on his head and walk away with him.