So during the last month I've continued tweaking the tool, adding some more niche features. I think all functionality should be covered now, which makes this a good time to post the update here.
Version 1.1.6 was the big Roman Numeral update that adds lots of functionality for handling front matter sections. You can either tell the book outright how many front matter pages there are as with the normal page count, or you can combine it with the page=>ToC map and really go nuts micromanaging.
Version 1.1.7 added mostly bug fixes and some more options for handling documents that are either not included in the ebook's "spine" definition or were designated as not "linear", as those were special cases that I was not aware of when I started this project but now they can be handled in a variety of ways.
Since I am now able to edit the opening post, and can just update its Attachments I also no longer need to duplicate them across many posts, so check out the new versions
there.
...Still thinking about reworking it into a calibre plugin in the future.