As to the barn, I never got the impression that it was on Gallifrey. The Doctor, or War Doctor or whatever, made it a point to park the T.A.R.D.I.S. well away so 'she' wouldnt know what he was doing, so it kinda makes sense that he woulda picked a secluded place to frag all of Gallifrey and the Daleks, so someplace on Gallifrey just doesnt seem to fit. But, I could be wrong, I'm hoping they explore this a little more in upcoming episodes, and as to why he has a familiar face, and how he apparently regenerates into Tom Baker again.
It is possible the "barn" may have not been on Gallifrey (but i think it was)
Gallifreyan children are brought up by their extended family prior to the age of eight, so he could have been anywhere before then,