Maybe this answer from Moffat will offer some insight:
Question from viewer:
In the time of the doctor the Doctor prevents his death in Trenzalore. But in the Name of the Doctor, Clara enters his Timestream in his tomb in Trenzalore. Surely if the Doctor prevented his own death, how would his tomb be there? And if he didn’t die, Clara would never have entered his time stream, and they would never have met in Asylum of Daleks, The Snowmen. So does that mean that much of the last series never happened?
Moffat:
Changing time is tricky. It’s a bit like a detective story; so as long as there isn’t an actual body, you have got a certain wiggle room – for instance, if the body has, rather conveniently, been burned in a boat in Utah.
Here is the thing: I can change the future so long, as the future hasn’t been already established as part of my own past. I can’t rescue Amy and Rory because I already know I didn’t.
But what do I know about Trenzalore. There is a big monument that looks very like my TARDIS. There’s a temporal fissure leading to my timeline. Maybe it’s my grave. Maybe, one day, that’s still where I get buried. Maybe, it’s something else entirely, and we all got it wrong. Don’t know. Don’t plan to find out for as long as possible.
The main thing is, Clara still jumped into my timestream, and still ended up helping me all through my life. All that is established, unchanged – but there’s wiggle room.