Kevin, the redlining in Preview was a thow-away line - maybe 'tin foil hat' doesn't translate
The lack of Ignore, … etc, make it less attractive, one the reasons I like the list based spell checkers is that I can deal with proper names - e.g. Pavlovna - by ignoring them, or adding them to a book specific dictionary.
I tend to treat the opf file as a black-box. Given my druthers I rather select a content file (text, image, etc) and get the corresponding folder in the Open With program - but I also recall you want to make Sigil's folder structure optional.
BR