I think you'd need to add Ignore, Ignore all, and Add to dictionary to the misspelling context menu.
Some users prefer to have a list of misspellings - there's a thread somewhere in which it was discussed, a consequence of which was that Toxaris developed something similar to Sigil's spell checker for his Word Add-in.
This maybe germane:
The Bowpad and Notepad++ instances were started by Sigil's Open With.
FWIW the tree in Bowpad can be turned on an off a'la Sigil's Book Browser, its a bit more awkward in Notepad++, you have to open the folder, rather than a file in the folder.
There maybe reasons, other than proofreading, that would make the ability to PageEdit multiple files in the same instance attractive.
Curious - how would Sigil keep track of which files PageEdit changed. At the moment if I open chapter01.xhtml in Bowpad, make changes, save, then open chapter02.xhtml, make changes and save, and then exit Bowpad; Sigil will only pick-up the changes I make to Chapter01.xhtml. That's okay, 'cause that's what's written on the tin - but if there's to be a multi-file PageEdit, why not multi-file any Open With?
I'd rather have red underlines in Preview
BR