Following the discussion in the thread
Generating mobi and KF8 parts of Kindle file from separate sources I have updated mobi_unpack to (a) create mobi7-only and mobi8-only variants of the input file in the case where it is a combo mobi7/mobi8 file and (b) properly handle unpacking mobi8-only files. The updated zip folder is attached.
I've tested with the kindle previewer and my Fire. The only wrinkle I've seen is if I try to open the mobi7-only file in the previewer with device=Fire it chokes. This doesn't happen if I open the mobi7-only file on my actual Fire. The previewer also works properly with the mobi7-only file if I set device=Kindle. This may be an issue with the zero-length RESC and FONT records in the mobi7-only files, who knows. I'm leaving that issue open because I don't think it's important and might actually reflect a bug in the previewer. Incidentally I don't have my Kindle3 with me so I haven't checked the mobi7-only files on it.