It's a bad book, for one thing. You can't have ampersands in a toc.ncx file. It needs to be an entity. But with recent versions of Sigil, the ampersand just get tossed out and things move on. That's why I wasn't seeing the truncation you were. I had no idea you were still using an old 0.8.x version of Sigil. Is it one of my deb packages or did you compile it yourself?
Either way, you're right; this isn't a plugin issue,
per se. I'll see if we want to make KindleUnpack accountable for heading off ampersands in ncx files in the future.
I might be able to cobble together a work-around for this in KindleImport, for the time being, though. I'll look into it.
Thanks for the persistence!
EDIT: actually it won't be as easy to kludge together a workaround for this in the KindleImport plugin as I thought. Might have to see about bouncing this upstream to KindleUnpack.