as always this is cardroom dependent. some will allow you in on the button others won't. more and more are now allowing players to play from the button or sb if joining a full table
the only seat you can never join and be dealt in is between the button and the sb, all cardrooms will make you wait in this position
nope if you come in there you guest (according to some cardrooms)
in cash yes, but I assume we're talking about tournaments here though, I certainly am anyway