If not defenseless the forearm to the helmet would not be a foul so B1 is out for a play.
Doesn't rule 9-1-2-a say that striking the helmet with the (extended) forearm is a foul at all times? Striking the neck area is where you have to rule on defenselessness.
"No person subject to the rules shall strike an opponent with the knee; strike an opponent’s helmet (including the face mask), neck, face or any other part of the body with an extended forearm, elbow, locked hands, palm, fist, or the heel, back or side of the open hand; or gouge an opponent (A.R. 9-1-2-I)."