Do you think these mappings would make sense for tv shows/movies?
I don't know about .nfos, but generally, in terms of integration with Mp3tag, unless there's a specific exception made (like I requested for TVSEASON), tags for MKV files will get created at level T=50 (since Mp3tag v3.28e).
This means MKV files with COUNTRY T=70 and TITLE T=60 tags could not be maintained at that level with Mp3tag.
If you have TITLE levels above 50 for example, they'll just appear against Mp3tag's TITLE field in its tag editor, and get saved as T=50 level tags whenever you try to edit them.
Multiple occurrences of MKV TITLE tags at different levels (or the same level) will appear in Mp3tag's TITLE field separated by double backslashes (\\), but will all be saved at T=50 level once edited.
For your other listed mappings at level 50, you can map them any way you like of course. But generally, yes, if you can find a corresponding official tag name in the Matroska spec, then you'd be as well to use that.
Naturally, this doesn't mean we'd then expect them all to display universally, everywhere, all at once, in any application you can think of. Far from it.
My interest has been in seeing what MKV tags I can get to display in MS Windows environment (I'm currently still on Windows 10).
So, my answer to the question whether to use SUMMARY or DESCRIPTION tag for example, would be to prefer Description, as Description will display in Windows, and Summary won't.
However, Windows actually displays MKV DESCRIPTION tag as Comments and also displays MKV COMMENTS tag as Comments (Windows property: System.Comment).
If both DESCRIPTION and COMMENT tags exist then Windows seems to prefer whichever one it finds last.
Because of this unpredictable behaviour, and the fact that Mp3tag mappings lists COMMENT rather than DESCRIPTION, I tend to just use COMMENT.
Windows displays DATE_RELEASED, and doesn't display DATE_RECORDED tag, so I've tended to prefer DATE_RELEASED too.
VLC player media information reverses this approach and displays DATE_RECORED as its preferred date however.
And Mp3tag's YEAR field is mapped to DATE_RECORED.
As with anything, it depends on the application, so you could populate them both, which is what I've started doing.
I tend to put just the year (eg. 2025) in DATE_RECORDED, using Mp3tag's YEAR field, and the full date (eg. 2025-02-28) using DATE_RELEASED.
I populate TMDB with just the id number rather than prefixing it with movie/ or tv/ as I find that way it plugs in neatly to API calls when using Tag Sources (something I've just started getting into).
As for multi-tagging, no, I don't use structured organizational tags, I just put all information in one tag instance.
One ARTIST tag for example, with comma separated names of the movie's starring cast.
I know Matroska supports tag nesting, but Windows doesn't. For the MKV tags that it does display, Windows tends to display only the last one it finds.
And, though Mp3tag will write separate tags if separated with a double backslash (\\), it doesn't support fancy sub-tagging.
For example, Mp3tag won't create CHARACTER tags (name of the character an actor plays) as sub-tags of (real name) ACTOR tags.