To try to answer my own question about the proper format of things, after reading some more:
Recording
- of video type
- artist is as specified in the title
- has the vocals and vocals arranger and other metadata expected from audio recordings
- if it is a new MV (as it happens often), should have the illustrator, animator and related types
- has the cover recording relation of the original song; for re-used MVs it can be assumed the illustrator is that of the original MV
- (?) does not include the streaming URL
Release
- same artist credit as Recording
- of video type and “Other” (could be improved by this suggestion, but it should not be a Single)
- has the illustrator of the thumbnail; possibly the same as the Recording
- (?) has the streaming URL of this release, possibly multiple if they are identical (i.e. some older entries are also officially available on Niconico/ニコニコ動画, but are the same)
- has the publish date of the oldest URL, in UTC timezone
- (?) does not have a set country
(Again, corrections welcome)
I am confident in this format except that possibly the streaming URL could be in both entries and that the country could be set to Worldwide/XW. As for having multiple URLs, the oldest URL is the canonical source and others are late re-uploads. If they are not identical then it might warrant a separate release in the same release group, but I do not think this happens.
I will also try to make reference of Twitter announcements for the publish dates (since YouTube does not tell me the exact hour to correct for timezones) and possible extra credits, but I don’t know how far in the past these go
Other than this there is some possible confusion for covers which reuse the original song MV, for example, ななひら’s きゅびずむ/きゅびびびびずむ covers only have an original illustration for the thumbnail, whose illustrator would be credited in the Release but not Recording