In fact, their appearances in First Contact, and especially in Enterprise, could actually warrant a continuity rewrite.
I don't see it that way. I view those appearances as a closed time loop. They were part of history all along. You can't prove that's not the case.
I'm not sure how the Borg could be worked into the Abrams timeline. They are still out in the Delta Quadrant, which is far outside the reach of even the Abramsverse Starfleet.