On your point #1, please see here for an explanation:
Support for new iTunes Grouping id3 frame "GRP1"?
UPDATE: I suspect that your reference above to that ID3.org
page is out of date. The Hydrogen Audio mapping tables do list GRP1 for Id3v2.3/4. Scroll down to the row for "Content Group".
On your suggestion for a "Produced Notice" TPRO frame for id3v2.4, I'll let others comment.
I never use id3v2.4.