TPTB have state that Matt is the 11th Doctor in mini-docs.
I think regens have to be complete transformation and that they are conscious choice. I'm sure there may be some explanation like the process must go to completion or something.
As for Time Lords dying and regenerating over and over that has to do with the time lock. They are forever trapped within a single point or small period or something. Just like Capt Jack can't die because he's "fixed".