On your one hand backhand video there might be an issue that could lead to Tennis Elbow.
If the wrist is held in flexion
(hand bent down) the risk of tennis elbow increases. If the wrist is held in extension
, as the pros do, the risk of tennis elbow decreases. This is according to references by Knudson. See especially his book discussion, Biomechanical Principles of Tennis Technique
, D. Knudson.
You can look at pictures of pros hitting one hand backhands and see that they do so with extended wrists. Amateurs more often do 1hbh's with their wrists flexed. It is claimed that as a result the occurrence of Tennis Elbow is much higher among amateur tennis players than among pro players.
Look at the extended wrists in these pro pictures.
Google: one hand backhand pictures
You have a special racket grip so it may not be the same. But probably the forces before and at impact and the degree of wrist flexion are the most important things.