When it comes to the Doctor he tends to come and go based on an emergency event, he seldom sticks around to pick up the pieces, he leaves that to whoever is there to do it themselves more often than not trusting in those that remain being able to sort out everything once the day is usually saved. He really doesnt care to deal with the minor stuff much.
And as for how time keeps changing, if you take into the old Who shows that had space stations and the like I believe its easy to say that history has been changed/altered several times over. Usually later seasons keep up with the years and what is going on but what has happened before and certain advances are forgotten about. If nothing else you can basically call these sort of paradoxes to be time itself correcting itself. Not sure if it was the last season or around the Pandroa episodes but the Doctor himself mentioned certain paradoxes resolve themselves. So with the giant Cyberman, the dinosaur, its just time basically going Nope, thats not right, strike that. and its a case of what comes later never remembering or knowing it had occurred.
You can think of it as someone shuffling a deck of cards, nothings to say what happened before is exactly wiped out depending if they need to drop back to it but it gets put within the deck for safe keeping until then.
And as for how time keeps changing, if you take into the old Who shows that had space stations and the like I believe its easy to say that history has been changed/altered several times over. Usually later seasons keep up with the years and what is going on but what has happened before and certain advances are forgotten about. If nothing else you can basically call these sort of paradoxes to be time itself correcting itself. Not sure if it was the last season or around the Pandroa episodes but the Doctor himself mentioned certain paradoxes resolve themselves. So with the giant Cyberman, the dinosaur, its just time basically going Nope, thats not right, strike that. and its a case of what comes later never remembering or knowing it had occurred.
You can think of it as someone shuffling a deck of cards, nothings to say what happened before is exactly wiped out depending if they need to drop back to it but it gets put within the deck for safe keeping until then.