I'm a little hazy on what you have in mind, particularly with regard to the current rather odd preset choices (the stuff beyond jAlbum and xmp). So I guess I'd have to see the result to offer an opinion.
Let me put it this way: Do you feel there is a need to have individual toggles for "jAlbum" and "xmp" or can both apply to both comments, titles and keywords?
Not really. But then you still have to deal with other metadata entities that one would apply to titles and others that would apply to comments. You can't "toggle" something like IPTC Headline, which is sort of a "title thing," not a "comment thing."
I would rather see a straightforward set of choices: what fields should be grabbed for title, what fields should be grabbed for comment, and what fields should be grabbed for keywords. Anything less than that, IMO, just perpetuates the confusion.
v34.2 now released. I haven't fully gone your suggested way with editable metadata fields, but now at least the "jAlbum" source has priority over "xmp" and both "jAlbum" and "xmp" are broken out of the "comment sources" box and now apply to all metadata (comments, titles, keywords, flags and ratings)
The UI checkboxes now corresponds to how things work in reality