If the "C" was always meant to go back in time, how do you reconcile the new JJ Abrams reboot? Was Spock and Nero always meant to go back in time?
Different writers different time travel rules.
If you're going by Back to the Future logic, that supports my conclusion even more. When Marty goes back to 1985 at the end of the first movie, things are vastly different. Not only are his parents now hugely successful, but the twin pines mall is now call lone pine mall. And at the end of Back to the Future 3, Clayton Ravine is now called Eastwood Ravine. Things may be back to normal, but it's not completely normal. That's what happened after Yesterday's Enterprise. Before the Enterprise C came through the rift, there was no Sela. When it went back through the rift with Tasha Yar onboard, there was.
Interesting point. Seems I destroyed my own arguement with my own logic.
I only recently spotted that they renamed it "Lone Pine Mall" - a fantastic bit of detail.
The truth is, time travel rules depend on the writer - so we have no way of telling who's interpretation is right or wrong. That Sela did not show up before Yesterday's Enterprise
is no indication that she did not exist before then. She may have done, she may not, we will never know.