As I said it's obvious the idea of Smith's Doctor being the last one is a fairly recent one, as a poster on Gallifrey Base pointed out.
Why is there a need to nullify his ability to regenerate when hes poisoned in Lets Kill Hitler?
Why does River Song tell him off for using some of his regneration energy to mend her hand in Angels take manhattan?
Why does he START to regenerate after the first shot he takes from the 'impossible astronaut'?
For whatever reasons Moffat moved the goalposts again probably because it's the anniversary year and he just wanted to set up the next 50 years.
Was it ever actually stated that the poison that River used on the Doctor inhibited his ability to regenerate? When the Doctor asks the TARDIS interface he is merely told that his regeneration is disabled without further clarification; that could easily be because he doesn't have enough energy left to do so.
Also the Doctor may not be aware of how many lives he has left, but River Song probably does having studied the Doctor's various lives, she just can't tell him. So in that scene she lambastes the Doctor for using what remains of his regeneration energy to heal her wrist.
The final one is easily explained as it is the Teselecta that is simulating the regeneration effect.