I know fiction has to work, so writers basically never deal with this, but man, the second Nora showed up, she basically should have evaporated out of the timeline. For example, if on whatever day Nora is conceived, Barry and Iris have sex one thousandth of a second later: no Nora. There are trillions of tiny factors that lead up to that moment. For that matter, and not to be gross, but a guy releases like 200 million sperm at a time. Just look at how drastically different fraternal twins can be, for example.
Nora changed things so much that there's effectively a 0% chance that she would be born. The fact that the writers wrote in the fact that she vanished from the timeline means now we're in that sort of gray middle area between "it just works" fiction and the reality that any slight change to the timeline would have erased her to begin with.
Also, Barry and Iris remember her. How's that work? I guess we're operating on Looper rules.