Posting a beta of an overhauled version (v0.79.79) that works with multiple selections. It also uses an experimental version of the core KindleUnpack code. If and when I make this the "official" version, it will become version 0.80.0
Multiple selection only works with the EPUB extraction from AZW3s or the PDF extraction from AZW4s features. You cannot "unpack" multiple books.
Books that do not have an AZW3 format will be ignored when selecting multiple books for EPUB extraction. Same thing goes for PDF extraction. If some of your KF8 books don't have an AZW3 format listed in calibre, you'll just have extract the EPUBs from those one-at-a-time.
If the target format (EPUB or PDF) already exists for a book, it will never be overwritten by the KindleUnpack plugin.
Check the log to see the details of what happened during a multi-book run.
EDIT: latest version of beta (v0.79.80) can be found in
Post #307.