No, Merlin cannot sell the land, Alton is not Merlin's asset to sell, it is a very simple concept. They (Merlin) could conceivably terminate the lease they hold and hand back the land to Leslau, or stop operating the theme park as Tom said, but those are completely different things to 'selling the park', what you are suggesting is me being able to sell the flat I live in even though I only have a lease on it, that can't happen. Nor was it a 'massive loan guaranteed on the land' as that would mean they still owned the land, which they don't.
The only 'asset' Merlin own in the UK is the land Chessington sits on and an empty field next to Thorpe, all the other theme park attractions are operated by Merlin, but not owned, there is a difference between owning something and leasing/operating it, a very big difference in fact. Granted they own the brand 'Alton Towers Resort' but that's not the same thing.
As for the last sentence, I'm sure a few years ago we would have the same level of confidence that engineering staff would know how many cars were on a ride, but alas we know how that turned out. All I'm saying is any talk of Merlin selling Alton is moot because they can't sell land they don't own, and I wouldn't assume they aren't locked into any agreements without seriously huge termination terms or restrictions on what they can do because in fairy tail land people do what they want, that's not how these deals are setup.