So, SR3 explicitly stated that you couldn't heal drain wounds via magic. However, I do not recall seeing that stipulation to SR4? So, anything (other than a house rule) to stop Mage A from overcasting and Mage B from summarily healing all physical wounds. Since "undercasting" causes stun which cannot be healed (but can be patched now) it seems like overcasting is almost a good idea as long as help is there when you need it.

I haven't seen it yet. You do have to be careful not to take too much drain as you only get one shot at magical healing clearing it up.

I do think the jury might still be out on whether or not Regenerating critters heal drain damage. It does somewhat leave it open to intepretation that drain counts as magical damage. I'd say yes, but it isn't as concrete as it should be.