You asked what should have happened?
Well,
Live ball foul for illegal motion against A.
Dead ball foul for personal foul againt B.
Play ended short of line to gain.
Team B has the choice of accepting or declining the penalty for A's foul.
Take the penalty it will 5 yards agaisnt A and then 15 yards against B, if the distance is enough for a first down, then first down for A, if not then repeat 4th down.
Decline the penalty and it will be B's fall 15 yards behind were the play ended for the personal foul.
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