I don't think the engine would coast to a stop until it was too late. You have the right idea about the fenders, they would cause the wheels to lift off the ground and prevent further rearing back.
Ferguson built a plow to use with the old Fordsons that had an upper link to stop the tractor rearing back. He further developed this idea for the 1939 9N tractors.
What most plow operators used otherwise, was a trip clevis, that was set for a certain breakaway load. If the plow hit a rock, it would break away from the tractor instead of making it rear back or instead of tearing itself apart. Later designs had variations on the trip-beam plow.