You definitely want Block here. Because this set is supposed to be a supprise, you don't want your trapping move to suddenly miss on you and reveal your set. Not to mention, the effect of Fire Spin won't last long enough (you need 3 turns to Trap, use Imprison, and Transform, along an at least extra 4 turns of Struggle damage. Fire Spin only lasts 5 turns max) while Block lasts untill they faint or you switch out. The struggle damage migates the need of Fire Spin chip anyway.
For Knock Off, run Soft-Boiled there. This allows Mew to heal and potentially attempt its gimick again.