Style generally operates under common sense being better than hard limits. There might be some cases where 15 artists do make sense. That Matthäus-Passion release isn’t one of them though IMO because, as you said, there are only two artists clearly credited on that cover way above everyone else (“only the most prominently featured names need to be included”).
Even if we did add everyone, they should be added as credited, which makes it 188 - still a lot but somewhat more reasonable (“Bach; Prégardien, Goerne, Schäfer, Röschmann, Fink, von Magnus, Schade, Schäfer, Henschel, Widmer, Arnold Schoenberg Chor, Wiener Sängerknaben, Concentus Musicus Wien, Nikolaus Harnoncourt”).
Basically 1 and 4. 2 is, as always, secondary (database comes over taggers), 3 is obviously useful in general but not specifically what we care about in this case (it wouldn’t affect length). The main use is that a release can be found from the discographies of any of the main artists credited for it.
I strongly disagree with this one. It’s not the editor’s job to count characters - it’s to show who the release is credited to. We already have a “use your best discretion to pick only the most prominent artists” guideline. That will sometimes lead to big artist strings, yes - but that’s because that’s the actual credit.
Is pretty much my opinion on this. We shouldn’t design our data so that a tagger is happy - we should improve taggers so that they can use data properly.