Seems like you have enough questions there for a short followup all by yourself! 

Just to hit a couple of random ones here (I'll probably try to follow up in audio in the "opus codec" episode once I've gotten caught up to that):

By "3 or 4 others" (metadata formats) besides id3 and vorbiscomments I was referring to RIFF INFO chunks (.wav metadata), webm/matroska tags, .wma microsoft-screwy-thing, and the undocumented special iTunes thing.

I did count right around 75 individual id3v2.3 field (/frame/tag) names, though I didn't go back to confirm the exact number.

Minor point - I need to enunciate better I think - it's "id3ted" rather than "id3tdd".

Good question about the geolocation - it's "any of those that are relevant". For photographs, the geolocation is always assumed to be "where the photographer was standing when the picture was taken" and there aren't too many cases where any other interpretation makes much sense. (In a telephoto of an obvious landmark it might make sense to geotag with the location of the landmark instead, or for an image of a map, it might make sense to geotag the center of where the map represents). As far as I know, photo geotagging only supports a single geolocation per image as well [not necessarily counting geoTIFF], so options are limited.
With vorbiscomments explicitly designed to support multiples of all tags, the way I think of it is you put in a geo_location tag for any location that is relevant to the recording: imagine that someone wants to generate openstreetmap (or Google Maps or whatever) pages with markers that go with the audio files. The way I figure it, the geo_location tags should provide the locations of all of the markers that the hypothetical link-maker would want to show. (It's probably worth proposing a "description" addition to the geo_location tag now that you mention this, though: something like "geo_location;35.1592;-98.4422;;Nowhere, OK used as example location"

Also, thanks for teaching ME something - I usually tend to think of playback as something that doesn't need an internet connection, so I feel stupid for only thinking of web links to relevant pages (what I used the "chapter###url" tags for, e.g. the id3v2.3 chapter's URL should have been a link to the specification online) and somehow completely missed using it to pull a slideshow from the internet while playing. Now I have to try that... (For me, chapter marks are more about having convenient "jump to:" points in the audio.) It's worth noting that doing "fetch pictures from the internet" like that also makes it a way to put "web bugs" in audio files...

(Finally: I should say I don't necessarily disagree with your contention of how one SHOULD use the "artist" tag...well, actually I do but less strongly and not because that's not what the specification says. The cases where it matters seem like they wouldn't come up TOO often in practice. The examples that come to mind are mostly things like "Darryl Hall and John Oates" and "Paul Simon and Art Garfunkel" and "William Gilbert and Arthur Sullivan", all of whom are so well known as duos that "Hall and Oates", "Simon and Garfunkel", and "Gilbert and Sullivan" are practically all single-word names (kind of like that "colladoody" video game people were going on about...) and makes sense as a "singlet".  On the other hand, there's "Ebony and Ivory" (Stevie Wonder and Paul McCartney?), where I think it would definitely be more appropriate to follow the specification and give each "artist" their own tag. I just think it's more consistent to use all of the tags the same as per specification, and more people using the tags correctly will encourage playback software to support it correctly.)