Shaymin-EX

If my opponent takes a prize and then I use Naganadel GX's "Injection GX" attack to add a prize to my opponent's Prize Cards, would my Dubwool V's "Revenge Blast" attack do only 120 damage because the number of prizes in my opponent's Prize Card area has increased? Or does it do 150 damage (120+30) because the opponent has already taken that prize (even though it is now back in the prize pile)?

Revenge Blast is based on the number of prize cards TAKEN, so even though the Prize Card gets put back into the opponent's area it still counts as +30 damage for Revenge Blast.

For Billowing Smoke, since prize cards are discarded when the Pokemon it's attached to gets KO'd, how does this interact with Dubwool V's "Revenge Blast" attack that "does 30 more damage for each Prize card your opponent has taken"?

Revenge Blast does not count any prizes that were discarded due to Billowing Smoke.