Twice the Doctor has been shot kwa a Dalek. Both time he's survived without having to regenerate. how could that happen when both time he was hit full on?

If my memory serves me right, in journey's end he actually start the regeneration process after being shot, but it didn't want to change, so once he was heal he transfered the regeneration energy to his "fighting hand" (wich is a compatible bio receptacle).
In big bang, he was dying but the pandorica healed him (like it kept Amy alive for 2000 years and healed her once young Amy touched it). And when Amy remember him he isn't dying anymore anyway because time has been rewritten.

Cos he is hard!

He was not wanting to regenerate,so,he did for his hand to regenerate!