So if he can't go to 1938 can't he go to 1939?
Hasn't the Doctor watched Bill and Ted. He could have gone back in time and put that gravestone there himself. If Amy can put her name on it, the Doctor could surely make it disappear. Or do a Back to the Future on it and it's erased. What's the point of having a Time Machine if you don't make good use of it. The magic headstone tells me Rory and Amy weren't originally part of that timeline so where's the paradox? It's not like Rose's Dad. It's not a fixed point. Or even consider the Butterfly effect of having two people from 2012 living in 1938 who never should have been there to begin with.
And we still don't know why the Doctor uses Guns now and kills people.
Edit. Not a bad episode. Again, not great but Ok.
I think having an episode with the Angels in it and the fact it was to be the Ponds teary goodbye it's not too much of a surprise ending for them.