Is the Adrenal GS Attack (Roll check for 0) a new step taken before the SM rolls for defense check (higher than # of GS in swarm)?

If it is all one in the same roll, can a reroll be forced by a facing SM with a support token?

From the wording I think it's just part of the defence roll - i.e. "if your defending marine rolls a 0, the swarm moves and attacks again". That would mean that - facing correctly - it could be re-rolled.