Carbon will bake itself onto the innards of the bolt carrier and the tail of the bolt if you let it.
Yep. It will form a crust there, but you don't need to remove it. It bothers a lot of people and it does no harm to scrub it off, but the amount of buildup is self limiting.
the first place the gases go in the AR is into the bolt carrier key. Pipe cleaners are about the only thing that'll work.
Once again, its a comfort zone thing- the gas key really doesn't need to be cleaned.