Couldn't they be arrested for attempting to incite a riot?
Depends, we went over all of this during the Milo thing, where the supreme court considers that sort of language protected unless there's an "imminent and likely" threat. There's a lot of room for interpretation as to what "imminent" means, so in theory one could say "We should kill all the Jews" and it's protected, but if you say "Grab that Jew and kill him!" it's not, because it's inciting violence in a very clear, immediate way.
At that point, if people actually take up arms the legality is probably the least of our concerns, and if they don't I'm sure they can slime their way out of it by saying it was "just entertainment" or something.
Depends on if Trump fires Bannon first, which seems fairly likely as impeachment/a resignation will still probably not happen until 2018+. Alt-Right would turn on Trump then.
Hard to say, Bannon definitely gets them riled up but how many people are just there because of Trump and would stick with him no matter what? The exact mixture of "Angry Orange" and "White (Grape) Power" packets used to make this particular rusty metal tub of Flavor Aid is hard to discern at this point.