So you can't go back to New York in 1938? Go to London and send them some plane tickets.
Or leave a message for them in an earlier year. Or send someone else to pick them up...
Although I guess it's possible that even interacting or communicating
with them in some way could be dangerous and could upset the timeline.
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?
Well this could be an ability that just recently developed in him, or that he just recently honed. Or maybe it's because River once had the same Timelord energy herself, so she's more conducive to it.