While we are clear that the first doc-toc is the toc when a fragment identifier isn't supplied, is this potentially problematic given that a sectioning element (article, aside, etc.) could have its own toc and occur first?

This probably falls into the highly unlikely scenarios and may not be worth worrying about, but just has me wondering. I'm not even sure there's a good solution without always requiring a fragment identifier, as the nav could be inside a section (i.e., we'd start to get into picking and choosing sectioning elements).