My personal theory is that since both Chekovs would have been born after Nero's incursion, something changed so that nuChekov was biologically not the same person as prime Chekov, as he was born later in this new timeline.
...For all we know, the parents simply swapped the names of their two sons, Pavel and Pyotr. And this time the older, smarter brother (from 2241) did not die in the hands of Klingons. But perhaps the younger one (from 2246) did...
As has been stated, there's no reason for this new timeline to "heal" itself, as there's nothing to heal.
Yet every time our heroes travel to the past, their interference fails to change the future in any noticeable way save for the single thing they came to change. Repelling a 2063 Borg invasion, with quite a bit of destruction and casualties, does not cause Picard or any of his crew to be unborn. That does call for quite a bit of "healing" - unless one goes by the idea that every instance of time travel is predestined and built in, and Picard would not have born had he
not gone to the past. Which can be interpreted to be pretty much the same thing, really.
What is different about this adventure? Only the failure of Spock to return to his present, in the 24th century. But in the standard format of Trek time travel adventures, the past universe Spock has interfered with will have to heal somehow to allow for the present to remain unchanged. This should not be dependent on whether Spock actually achieved positive or negative interference, or whether he returned or not.
Timo Saloniemi