So you can't go back to New York in 1938? Go to London and send them some plane tickets.
Not without destroying New York/the World, as he clearly explained. It's less that he can't
go back, it's more than if he does
the risk of creating another major paradox is too great, resulting in a massive backlash.
So the Doctor can just dish out regeneration energy to heal people whenever he likes? Aren't there a couple of thousand times in the past fifty years of stories when that might have come in handy?
No, he can't. But just like River gave all of her regenerative energy to a fellow Timelord, he apparently can do much the same. If you're going to be upset about anything, it should be that he has that much fine control over it considering how piss-poor his control has been in the past compared to other Timelords.
That said, it's amazing how collectively and irrationally
hate-filled and racist (nationist?) British folks become. It's downright amusing, even.