Regarding #2, Ive seen a number of strange positions for the off hand when firing one handed. Side of the face comes to mind.
I don't know how much it would accomplish in defensive/combat shooting. I guess its better than letting it swing caveman style on your side. Personally I would be backpedaling and trying to get my second hand on the gun.
There's some portion of it that has to do with High-stress weapon retention. Its beyond me. I guess if someone reached for your pistol you could judo them off with your left hand. Like I said before, I'd rather just backpedal and fire, but I'm NO professional.