Thanks for the feedback!
Hmm, I felt that the increase in quantity was a stronger effect than reducing the chance would be. If it is too weak, which it might be, then I might bump up the chance to 30. I also considered making the chance increase over time-- 20% on turn 1, and then multiply by 1.2 each successive turn.
For your second point, each target is independent-- each has a 20% to be destroyed, and the chances are calculated separately. Added to notes.