I was unclear: I meant to look at the epub conversion, not at the fb2 original. Decoding the fb2 implementation of XML is for someone with a lot more patience and spare time than I have...
Since my understanding of your comments is that the TOC seems to be "broken" only in the epub, that's where we need to look for signs of breakage.
As a potential, kludgy solution: One way that print-published multivolume references "solved" this was by having only a "summary" table of contents in the main volume, with the detailed table of contents for volume I in volume I. Thus, you might want to try:
- convert to an epub WITHOUT generating a TOC
- split the resulting epub (using the epub split extension) into smaller, logical pieces with no more than about 500 "chapters" each (trying to keep below 512)
- for each of the split epubs, force it to generate a new TOC, but only after it's a separate epub; I prefer epub-to-epub conversion, but theducks suggested adding to the toolbar internally
- if you reallyreallyreally want this all in one epub, use the epub merge extension to merge them, but WITHOUT generating a new table of contents
- manually add a new summary table of contents (using the new file command in the editor) that just hyperlinks to each of the tables of contents in the individual volumes, or perhaps to the really major headings within each