I also have to change my mind on changing my mind. If B's BIB ends up being accepted, the enforcement spot is the 20, not the spot of the foul. Basic spot for enforcement after a touchback is the 20. It's a defensive foul (since A has the ball now), so enforcement is from the basic spot.
I pledge to not change my mind again, but I reserve the right to rescind this change of mind.
SO WHAT'S THE CORRECT ANSWER, ALREADY!!!!!!!!!!