Thank you so much for taking the time to figure this out. I could not find an answer to restore my editing pane!
Your solution, which is totally random, WORKED.
For others: The weirder thing is nothing appeared at first in my quick notes. It said there were things in there, but it was blank below the date titles. I stretched the second column out and waited. Eventually the images showed up. Failing them loading, you could add a quick note and then use the same technique. Open the new one, then quit the app and restart. The editing pane will suddenly appear!
Honestly, I don't even know how these came to be. I can see that the same weird image of my first quick note also appears in the lower left corner of my laptop screen. I'm too old for these newfangled dealbobs.
This must be a bug introduced via Sequoia 15.0. I see that there's an update for Sequoia 15.1, so I will update my software later today.