Re: editor
At the risk of resurrecting a dead horse, I do recall that in the runup to Sigil v. 0.6.0, ckeditor was tried as a "previewer." As elibrarian indicated, it could also be used as an editor, though Sigil wasn't set up to use it that way. Eventually, ckeditor was dropped for 0.6.0 (IIRC) for reasons too deep for me.
But... perhaps if we're starting more or less from scratch, we could use ckeditor as a starting point? I speak of what I know not, so maybe there are fundamental problems I'm unaware of. (and "what's this 'we' s**t, white man?" anyway...

)
BTW, as I remember it, the ckeditor interface in Sigil 0.5.x looked a lot like the present previewer. But I searched all my machines which have 0.7.2 and above installed, and can't find the ckeditor_config.js file so I suppose it (ckeditor) is not presently in use.