Some scientist belive that every time a decision is made an alternate world is set up where the opposite decision was made.

As far as Buffy. I don't know that it reaffirmed anything for her. I don't think she would have leapt unless she was sure it was going to work, so slight revision from above. But I also think she was through. She would have kept going out a sense of duty but this was the perfect out for her.