Interesting this thread came along. My nephew does not have a right hand (amputation) and wants to shoot. One goal of mine for the summer is to shoot a few matches and many practice sessions left handed only, using a rig set up for his needs. I need to get that skill set down so I can work with his issues.
As regards the chest grab with the non-firing hand, Tom Givens made the point that if you could get that second hand to support the firing hand or arm, do so. But for his one handed drills, he wanted the non-firing hand down by the side to simulate grievous damage.
Jeff Gonzales stated something to the effect of, if you can hold your hand against your chest, you can get it on the gun.
My own experience with taking a round to the left forearm was that I had zero use of that arm. It was dead weight.
I've broken a lot of bones in various endeavors and very rarely was that appendage able to support itself after suffering trauma (with the gun shot being the worst).
With regards to One Hand Shooting in general, an instructor once told me to cant the shooting hand slightly inward to give a more natural alignment of the arm and therefore steadier hold. Any input on this technique? Just thought I would mention it because I hadn't seen it mentioned yet.
Seems to be a matter of individual body mechanics. Works for some folks, not for others.
IMHO, best to try for yourself and see what works.