One other thing to try is to use ePub3-itizer v0.3.6 and when first invoked, look for the rocketship icon in the Finder's panel.
See this post to see what I am talking about here:
https://www.mobileread.com/forums/sh...&postcount=202
Click on that icon. It will force the Python file save as dialog to the front so you can see and interact with it.
Does that help?