@user_none: I fully understand the reasons you chose to not rewrite Sigil in python, I would probably have done the same if I were in your shoes. I was just saying, it's a pity
Regarding folding it into calibre, or alternatively, adding a GUI based ebook tweak tool to calibre, I dont imagine being able to directly use Sigil's code, it would need to be re-written. However, calibre has lots of ebook related code that can be re-used, for example, the ToC editor and the Polish Books code. This would make various bits of the re-implementation available for free and would have the bonus of supporting both epub and azw3. The idea would be to take inspiration and techniques from Sigil, not use the code directly.
I would be happy to help with such an effort, unfortunately, it's not something I am in a position to do alone, as my plate is pretty full with work that is a higher priority for calibre.