I don't think WB would recast Superman in the JL movie so i am pretty sure that they would have Cavill reprise his role as Supes I would also like Bale to return as Bats and i wouldn't mind seeing Reynolds return for continuity's sake
Maybe for ten minutes until Batman takes over...
Yep, it wouldn't sense.
Cavill is going to be Superman for a long, long time (assuming Man of Steel and Justice League are financial successes).