Other mistake: Just before the Doctor ignites the atmosphere, Sylvia's house is getting dangerously full of gas. When Sylvia and Wilf go outside after the air is cleared, the inside of their house is completely clear. Meanwhile, back at the Rattigan Academy, there is still gas lingering in the air. (00:34:43 - 00:37:15)
Other mistake: When the Earth's nuclear launch sites are being brought online, country by country, in preparation for a strike at the Sontaran ship, Russia is puzzlingly omitted, even though North Korea of all nations is included. (00:17:20)
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.