I like the idea, though I also already really like classical options such as footnotes, and don’t even mind footnotes on footnotes.
One UI feedback point: when expanding an item, it is currently shown exactly like the main text. I’d have preferred some form of highlighting, maybe slightly different font colours or background colour for each level, to show where the expansion starts and ends. I found when expanding an item, I wanted to read the addition but couldn’t quickly identify where it ended, which made it more difficult for my brain to selectively read just the expanded part.
Yes, definitely the WingDings. I got hit by the safety filters too, several times, until I asked Haiku to give me a text file of the story with all WingDings translated to normal font. Providing that text file to Opus worked without issues and we managed to have a good discussion about it.