he didn't do a thing about those mistakes! It was just a line that sounded good, apparently.
To be fair, in the episode the line seemed only to be about the Doctor deciding to make clear the nature of his relationship with Clara. It was the trailers constantly using the line that made it seem like it was setting something up.
Here's the thing: I don't want them to find Gallifrey. At least for a long, long time.
Lets face it: The Time Lords are a crap species. I mean, besides Romana, is there any other reason to bring back the Time Lords?
"This is Gallifrey" is a nice tune to listen to.
No, you're right, the Time Lords are more interesting in their absence and if Gallifrey is never found, you'll hear no complaints from me on the matter.
There was no evidence before The Time Of The Doctor that the Doctor was on his last regeneration,
That's because prior to Time of the Doctor, even Moffat didn't know the Doctor was on his last regeneration. The original plan for Time was that in addition to the truth field the Time Lords had set something up on Trenzalore preventing regeneration, thus requiring the Doctor to get really old, until finally after Clara lectured the crack, the Time Lords decided to restore regeneration. But since Day of the Doctor introduced a whole new previous incarnation of the Doctor, Moffat realized that by deciding the aborted regeneration from Journey's End counts, the Doctor could be on his last life now and they would have the regeneration preventer with a canonical precedent. And the Time Lords would end up granting the Doctor a new regeneration cycle, thereby putting that matter to rest also.