Simple, they put him on the boat with Dr Sheehan and just had the boat come dock. At the end, we can see his delusions seemed have a "reset" period where he likely wouldn't question how or why he got on a boat.
The gun was fake. They show its just a toy at the end. His delusions just convinced him it was real.
Good question. There's no reason to give him a gun at all (real or a toy). And here's another question...
If it was just a toy, then why would they demand that he turn it in?
Also: We never saw any "resetting" the entire movie. If so, we would have seen him back on the boat at the end (or at least in his cell thinking that he was back on the boat).
Come to think of it, we never saw his cell in Ward C at all. If he had really been a patient the whole time, why didn't they just take him to his cell and show him all of his things to prove that he'd been there for 2 years? No silly role-play required.