If a timeline is set in stone, then time traveling should be impossible. Period.

<-------A-------B------->

This is our timeline. If it is set in stone, then it should be immpossible for Ultimecia to go from point B to point A and give her powers to Edea. Anything else would be self contradictory. You can't travel back in time if the timeline is set in stone. Either all time traveling to go back and change the past is allowed, or none is. You're arguing that only certain events would be allowed is flawed. Of course, this again goes back to the fact that fate can prove anything. "Ultimecia was able to travel back in time and hand off her powers to Edea because she was fated to do so". Yeah, there's no way to prove/disprove that, technically. Logically, however, you offer no explanation as to why Ultimecia is able to change the past when Ellone isn't. You just say that Ultimecia is "fated" to change the past, and Ellone isn't. Some arguement. If you can't support it with any logic whatsoever, I'm not going to bother arguing with you about it.