Given how long it's been since the more recent chapters have been released this likely won't matter much as y'all already know the answer, but the real reason she went into the past was two-fold. 1) To ensure the past events actually play out properly so the current present can play out (wouldn't have happened otherwise if Yvonne
didn't save Elsa); and 2) acquire information. The previous Yvonne didn't have any clue about why her mother was alive in this timeline, why she herself wasn't being taken over by dark power like her story counterpart was, and why Elsa was so hung up on both Yvonne and some supposed promise. The biggest thing that Yvonne learned is that it was the Demon Lord's curse that was the biggest issue here, and now that she knows this she likely knows that the "system" wasn't some kind of game system trying to bring the story back on track. It was the Demon Lord's curse trying to make Elsa's life a living hell so she would succumb to the curse, thus releasing it. And that is the crux of this entire situation.
It was never about changing the past. That was just a lie Yvonne was told to ensure things went properly. It was about ensuring the present can happen and gain enough knowledge so they can survive to see the future.
Now for me to see if my conclusion is correct. More Yuri!