@KevinH
Unrelated to (and regardless of) what I decide to do for the newly opened epub's temporary name, I think we need to examine KindleUnpack's file naming methods for mobi-only kindlebooks. We're technically creating invalid OPF/NCX files whenever the original file name has an ampersand in it. Which means I'm, in turn, creating an invalid container.xml file when trying to man-handle KindleUnpack's output into an epub structure with this plugin.
It's either that, or I need to sanitize all filenames with my plugin (when unpacking mobi-only kindlebooks) and replace all references to them in all the KindelUnpack-produced files with those sanitized versions. A thought I don't really relish, needless to say.
The KF8 portions of Kindlebooks, don't suffer this file naming "problem."