The actors that do FFL come from other areas and are ranked into it and on contracts the same way the actors are for characters, other shows, etc....
That show can close and their contract can be dumped with no notice. If the show exists they get their hours, if it doesn't, they're SOL. More than likely they get shifted back into other roles. Most of them are Marvel heroes as well if I recall properly.
Anywho... I suspect the London facade will be extended. I'm not 100% sure the ride won't be entered via Diagon, or rather, somehow kept distinct from the usual park.
The side of Diagon that is near Fear Factor is all backlot type sets with no infrastructure really. I'm not sure they'd miss the opportunity to put happy riders into that shopping area by connecting them somehow.
Maybe Ministry adds a London park area between the two and a secondary entryway to Diagon, behind the 13 Grimald Place facade.