Great news about Moffat, though I think his "Silence in the Library" isn't being as well accepted according to the reviews (which I haven't read carefully for fear of spoilers)...
Regarding Regenerations....
Remember that the Valeyard means that one is already used up.
And... supposedly, the red-haired doctor is somewhere in the future.
Quatermass wrote:
12 regenerations, 13 lives. But remember, the Master was given a whole new life cycle (a thing offered to him in The Five Doctors).
How exactly were they planning on doing that? Burning up a sun for him? Handing back the black hole?
Anyway, the Master hasn't had a proper regeneration in years, not since Deadly Assassin. He's been hijacking bodies since then using something that his alterered DNA was capable of. I can't see how the Doctor expected him to regenerate at the end of Season 3. He knows he can't.
The best answer for the Doctor is the reminder that he's not just a timelord. He was something before he was a timelord.
I love that scene in Lungbarrow where it describes the Doctor's birth...
Gallifreyans were sterile and the Doctor was on the "Loom", an artificial means of creating offspring. Some formless thing came and in a flash of lightning jumped into the body on the loom.
When the doctor was born, they called him snail. He had a belly button - which other children of the loom did not.