So I guess right, Yvonne only recovered her memories of her previous life, knowing that this was the game world since enrolling at the magic school. And like the motif of reincarnating into a female villain in Chinese novels, she started to stay away from the female protagonist (Elsa), until the system appeared forcing her to bully Elsa.
As for Yvonne not remembering a few days into the future, that's normal, children forget quickly (at that time she hadn't recovered her memories so she was just a 7 year old girl). Besides, when she came to the future, she was only playing around in an adult's body with a woman whose face couldn't be seen clearly, so if she remembered it, she would only think she was dreaming at that time.
I'm a bit confused. By "a few days" do you mean after going back? Or the time spent displaced? If latter, it was 2 months, as per future lady's own words. 2 months is a lot of time to ingrain memories. If former, then I guess it's possible, but also kids that age tend to be beyond the easily-forget stage, into the proper developmental part of making memories and learning habits.