Footnotes on steroids...

I like what I see in your demo. These are most critical for our clients' work:

• Create my own (or the author's) hierarchy of footnote numbering.
• Combination of two or more different footnote "threads" in the same INDD file, maybe 1, 2, 3 and a, b, c and *, **, § with each series keeping track of its footnote members.
• Ability to continue footnote numbering across independent threaded story frames within the INDD layout file. Right now, each independent threaded story restarts numbering at "1".
• And, of course, full accessibility in the exported tagged PDF.

There are a few options and variations I'd like to see (just maybe, footnotes by column), but I think you have to back up to actual style and usage here, instead of what might be technically possible. I work a lot with 'scholarly' publications and can't say as I've ever seen horizontally grouped footnotes, or a mix of those and full-width ones. I would question the experience and reasoning of anyone who wanted to use such a nonstandard, visually complicated (to the point of confusing) layout — considering that footnotes are only used in fairly formal, 'upscale' writing that has many other guidelines and standards for consistent presentation.

Put another way — this kind of layout could have been achieved from the lead-type days onward, and can be done right now if you more or less manually manage it. And I'm sure there's a contingent who thinks it's a good idea to kick over all those musty, fusty style guides and do it some exciting new way. (The same contingent who thinks "move fast and break things" is a useful maxim.)

The TL;DR here might be that publication styling is not something held back by technical limitations; the implementation in Word, ID and every other publication tool reflects the needs and requirements of a long, long-standardized presentation approach. Consistent readability, across fields and decades, is more important than individual authors' notions of how to arrange notes. Style needs drive the tech, not vice-versa.

But if anyone's going to work on it, getting both foot- and end notes to quit being so fraggin' fragile would be nice. 🙂