Continuity mistake: When the Doctor overhears Alfie crying while stirring something in the kitchen, he has the spoon in his hand in the close-up, but his hand is suddenly empty in the wide shot. (00:23:02)
Continuity mistake: When Craig enters the house and falls back when the cybermat is attacking him, his arms are down by his sides, breaking his fall. As he lands, he's suddenly holding the mouse so it can't bite him. (00:25:51)
Revealing mistake: When the Cybermat gets free after the Doctor tries to hit it with a pot and accidentally gets Craig instead, it is very obviously CGI when it slides along the floor and stops by a piece of furniture. (00:26:34)
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.