Even with the resetting to the original timeline which inspires Eobard to change the timeline, it still results in a fatal paradox and not a causality loop. The alternate timeline (the one in which the show existed) is always voided. Hence the giant rift in space/time cliffhanger. That timeline likely can't exist in a persistent state and season 2 may be dealing with a broken timeline rather than a simply reset one (the pre-Nora's death timeline). Which could be backed up by Grant Gustin saying they will explore multiple timelines in the next season.
Your theory about one of Eddie's ancestors being killed to assure Eobard never exists to come back and kill Nora is a little off also. All you need is to ensure that Eobard doesn't exist in the original timeline also. Eddie (or any descendant pre-Eobard) dying in the original timeline would also ensure that. It's just that in the original timeline there is no cause or foreknowledge to make that possible.
Just copying this post ofmine from another thread, where I've acknowledged these points.
People said Eddie dies so RF can't exist to kill Nora. This creates what is known as a "Grandfather Paradox"
To explain, a GF paradox says if a Grandson was to go back in time and kill his grandad, he himself wouldn't exist. But if he doesn't exist, he can't kill his grandfather, meaning at some point in the future, he will still be born.
That's basically what would still happen to Reverse Flash lol.
He can't be born, he can't go back in time to kill Nora.
He can't kill Nora, she lives and Eddie doesn't have to make any sacrifices, meaning the Thawne bloodline will still thrive at some point in time, leading to the birth of Eobard in the future, meaning he can travel back in time and still do the deed eventually.
The Flashpoint Paradox events might play out, but still lead to the existence of Eobard Thawne.
If you think about it, if present day Wells/RF did get his speed, he could travel back in time to the night he killed Nora, meaning there would technically be 2 Reverse Flashes in one timeline.
It's why when Barry goes back in time, his future self will always be there. It's like BTTF part 2. Marty has to go back to 1955 AGAIN! When he goes there, the Marty from BTTF exists within that same period because he's already been to 1955, so that event still had to play out. Hence why I believe the murder of Nora Allen at the hands of The Reverse Flash WILL still happen.
As I said earlier, there are multiple timelines at play, it just depends on what happens. I still feel the BTTF2 explanation with the 2 timelines is the best in terms of sci-fi time travel lol.
Another way to look at it is that if Eddie kills himself and doesn't have any heirs, RF doesn't get born. SOOOOOO maybe a paradoxical RF still exists at some point in time to commit the murder. I still believe it's the GF Paradox at play with regards to Eddie and RF.
If you think about it, while RF was marooned in "our" timeline, Eddie grew up not knowing that his descendant was inhabiting the same timeline. RF didn't keep tabs on Eddie to ensure that they both survived, he knew he would. Obviously Eddie's sacrifice means he died before he was meant to.
IF Eddie had died when he was much younger (say before the night RF was meant to go back and kill Nora), I don't see how RF could travel to that night, if his ancestor had already perished. If he was too young to have any heirs, RF couldn't exist at any point in the future to attempt to come back and kill Nora. So this theoretical timeline would be where Barry has both his parents and Adult Eddie never existed.
The anomalies in all of this are the intervention of Future Flash plus Eddie's sacrifice.
LOL!
