① Finding the other releases for those is, errr, very difficult. The Power Classics discs, for example, do not have ISRCs on them. (The Brahms ones do, though).
Also, following an "only on the cover" rule would leave the composers on, e.g., https://musicbrainz.org/release/5fe76d6d-5b23-49c7-946a-ad922ccbcaf6 or https://musicbrainz.org/release/848c66a1-9374-4fc6-8dfd-157c8d7bff93 uncredited; that surely is not what we want... And speaking of that first one, the pianist on Totentanz is quite important to the sound on that track, shouldn't he be an artist on it and on the recording?
② As I pointed out before, I'm pretty sure that we're just trying to shove classical into a pop-based schema it doesn't really fit in. That's not a good way to design a database at all. Obviously, we shouldn't do that, except of course that so much stuff would break if we didn't. You're suggesting we switch from one way that doesn't fit to another, presumably because it's breaking something you care about. I'm pointing out that well, actually, you'd be breaking a lot of other stuff. There is a tradeoff to be made.