The problem that I have with time travel and time/memory altering themes is that they have always seemed to me to be the lamest of all "fixes" for whatever problems that the writers have gotten the characters into. Buffy, Willow, Spike, Angel, and all of the others have, either through their own or someone else's design, accident, stupidity, or simply fate, progressed to this point. Joss jumping up at this point and yelling, "Do over! Everybody back to Square One," isn't a solution, it's a cop out. Plus, unless the characters retain the knowledge of the original mistakes that they made, they will commit them once again. If they DO, however, retain that knowledge, and do everything differently, they run the risk of creating untold anomalies, almost certainly with worse eventual results that what had happened the first time around. (Look at "The Butterfly Effect," or "It's A great Life." as well as several "Twilight Zone," episodes to get my meaning. Either the course of time is fixed and unalterable, or it is in a state of such delicate balance that the least change will cause cataclysmic changes)
Joss is too talented, creative a writer to have to rely on such a crutch as time travel to solve Buffy's problems and straighten out the Twilight mess, regardless of who is ultimately revealed as the one responsible, whether it be Angel, the General, Amy, Warren, or someone we haven't up until this point heard of. Mistakes have been made, crimes have been committed. Someone is responsible, and that someone HAS to be made to pay. No parallel dimensions, no evil twin brothers, and no more "I Will Remember You" reboots.
And most certainly, no time travel.