the ribcage bulge is a constant problem - there are a few ways to combat it - with deformers etc. - one way is to build the arm at a 45 angle instead of straight out or to build it straight out - then move it down to 45 and tweak it and then bind it. The deformer idea is not so different from the flexor idea for the arm. this is more or less theory from me because i haven't accomplished it yet myself.

You should be able to copy and paste the animation to the other hand with no problem - if it's set driven key you can duplicate the hand group on the - 1 x axis to mirror it and the attributes should copy as well if you have inputs checked.

sorry not to be a little more specific - I'm on my way to work. This is just to point you in the right direction