For the first part, yeah, I didn't model any situation for "body in shock but still functioning despite lethal trauma." That's a pretty interesting and dramatic one, although I'd think it should be very rare. I do know some game systems that make this almost the default for PCs who are killed. Yeah, you're going to die, but you get one last action to make it heroic.
For the second part, It might work, but rolls can be very swingy in this game, and it would make Stamina and CUF very, very strong. You'd probably end up wanting to revise all NPCs, since by default almost none of them have any rating in the Stamina skill. The numbers also don't work out great for getting those big outcomes. If someone is lucky or has high ratings in CUF/stamina, you almost need to be doing 6 damage to kill them, which restricts it to pretty much strong hits with HMGs and the like. It also makes every combat roll an opposed roll... one character rolls to hit, the amount of hits they land becomes quite important, and then the target has to make this defensive roll. That makes it even more swingy, and takes away one thing I really do like about this system: damage rolls are part of the to-hit roll itself! (of course my own table here violates that, but it's always the same roll, no thought or adjustment generally required)
The more I think about it, the more I don't like what that rule would seem to do overall... but if you try it out, let me know how it goes!