This is a time travel story from beginning to end.
Keep in mind that current-Elsa is different from original-Elsa while reading this, with the original being the witch as we all know by now.
The one current-Elsa had her first kiss with and made a childhood promise to see the lights during the festival thing was a timelooped Yvonne whose memories are incomplete.
STAY WITH ME ON THIS, I promise it'll all make sense.
It all started with original-Elsa failing to save the world from the infinite waves of demons so she started time traveling over and over, trying to alter the future in some way. And that's where she started finding out about how the world always converges on certain events happening no matter how hard she tries to stop them.
This is (THE SYSTEM) that is controlling this otome game world, and the same system that is appearing before Yvonne.
So after failing over and over to save the world from the demon waves despite being able to time travel, original-Elsa's uses her insanely OP magic to bring in the MC's soul from another world, a being outside the full control of the worlds system, into Yvonne. An anomaly.
Think back to the nuns comment of "if anything were to fully posses her she would lose her soul". This indicates Yvonne has been made a vessel for another soul. That's why she's referred to by the nun as "a special being". The nun is obviously in kahoots with the original Elsa since she went with the "her memories are incomplete" mumbling.
Also the demon king not being the same gender as the isekai'd Yvonne knew it to be further indicates meddling by time traveling, not to mention the fact that she ended up at the same place as Yvonne and current-Elsa.
Yvonne is special because she's an outside force to (THE SYSTEM), and she's shown repeatedly throughout that she can circumvent it through her own will which would not have been possible if she remained the original-Yvonne.
So with the isekai'd Yvonne in place, original-Elsa is manipulating the world through timetravel. Maybe she's done this 10 times, 100 time, 10.000 times-- who knows. The goal is for past-Elsa to break the world system and become stronger than original-Elsa, strong enough to break the demon waves and/or (THE SYSTEM).
And the only to do this is to put past-Elsa and (THE ANOMALY FROM ANOTHER WORLD), Yvonne, in close proximity.
Hence the childhood promise. It's all manipulation from original-Elsa to diverge more and more from the original timeline with Yvonne as a focal point, to save the world from the demon waves.
Yvonne gives every indication that she actually has lived through everything from the start, learning swordsmanship, archery, dragonriding etc. So how did her memories "become incomplete"?
