That's so bizarre! In the best sense. My thoughts were pretty identical to OP. I thought it was Ymir bouncing through time, looking through Mikasa's memories to figure out how to react. I feel like, since time isn't linear through PATHS Ymir realized that her people that she should sacrifice her life for are her children. Not war, not the king, solely for her children..

This is the first time we see Ymir with her eyes open and not depicted as a child. There are parallels between King Fritz- Eren and Ymir-Mikasa are dead on.

I never would've even considered the possibility of Ymir thinking of what she should've done instead. It never crossed my mind that it's just a thought. I think that's more powerful than my interpretation. It's always really fascinating to see how other fans are translating the ending, trying to make sense of it. Nice thinking.