Sorry about that, I'm having a rough day and this seems like such a trivial thing that I am amazed we ended up going 18 rounds over it. I just didn't expect to have to spend hours arguing over it.
As for getting Sigil and calibre to co-operate I'm all for it. In fact I have been planning to add a button to the calibre GUI to launch Sigil with the EPUB in the calibre database (making for a convenient workflow). With regards to that, I have some ideas about IPC I'd like to hash out with you, but perhaps over email. Just PM me your email address.