[enuddleyarbl]

Your line numbers are different from mine, but I'm almost positive those are the nbsp things I had wondered about earlier. Interesting that simply removing the italic formatting from them would allow the book to convert properly. Searching through the book, even though that code for an nbsp is used a LOT througout the book, that clump in part0035 seem to be the only ones that are styled with an italic.

EDIT: I also confirm your finding. I removed all five (?) italic-based spans around those nbsps in part0035 and the resulting AZW3 converts to EPUB3 without problem (even with heuristic processing on and doing the "Ensure scene breaks are consistently formatted" option). Good catch.