"a) If they wanted to go outside of the white male regeneration they should have done it long ago, not when we are into double figures. A couple of doctors into the reboot would have been the time to do it if they wanted to."
Well, huh.
They can't do it because they haven't already done it is not a strong argument IMO.
They changed the sex of the Master on the character's first regeneration/second body of New Who.
The first non doctor on screen regeneration of New Who was white man to black woman
And the doctor changes his apparent age with nonchalance, the change from Smith to Capaldi being, what, two decades?