Nigol, any interest in adding a new edit option in ePubFixer on the main screen, namely an "Edit Text Files" using any user-supplied text editor?
I have used tweak_epub
(part of the Atlantis Word Processor) and it's simplicity is it's strength. It just opens the .epub, let's you change some text using your own text editor for any file therein contained (except images) and then saves the .epub.
It's useful for deleting/editing a CSS style that doesn't work or changing some metadata info. It doesn't pre-parse nor post-split like Sigil, but it get's you (quickly) right to the text you want to change and move on!