Hello @dreieck and thank you for that well written report.
Unfortunately both your suggestions were not only quite difficult to implement, but also may lead to a box-of-pandora infinite-nesting type situation. In my opinion, nested footnotes are suboptimal UX-wise and should not be allowed.
So I’ve opted for a third route, which is to strip away any nested footnotes.
I know this may not be what you hoped for, but it at least addresses the bug of pretending it works while it doesn’t.
If you’re convinced this functionality is worthwhile having, I’d recommend adding your vote to this existing feature request, asking the same thing.