does cramming more music on a CD actually affect sound quality (as it did with vinyl) other than that they become more skip prone towards the end?
So long as it's Red Book standard: stereo, PCM, 16 bit, 44.1 kHz sampling rate, 80 minutes of audio fits on a standard 700 MB CD. In order to be sold with the "Compact Disc" logo, all commercial CD's must meet this standard. Using various compressions (mp3 etc), can save space, but because manufacturer compatibility standards and copywrite law, cannot be sold as a commercial musical release. There are various sound effect libraries and other sample media that do use mp3 on CD's, but these are explicitly labelled as such.
The problem with CD remasters of older material usually boils down to the competence of the mastering engineer. In the early days of CD's, analog master tapes were dubbed to Sony U-matic 1630 digital
videotape machines, which included a built-in upper midrange EQ emphasis curve and a VERY shitty sounding AGC (automatic gain control) and the CD's were produced from the resulting digital tape. This practice continued until the early 1990's, when record labels began using dedicated audio digital format masters. That's why there were so many remastered albums that were released at that time, but many were 'rush jobs,' employing less-than-stellar (to be kind) mastering engineers supervising the transfers from the original analog masters. Many of these so-called "mastering engineers" were label coffee-jockeys whose only qualification was being able to thread the machines.
Modern masters now take the other tack and are often made at higher resolutions and then dithered down to CD format. IMO, this has its drawbacks as well, but is the industry standard.
In the case of a remix and remaster, which is
completely different, the competence, motivation, ears, and ego of the producer summarily to that of the mastering engineer affect the finished product. There are lots of guys with enormous credibility that have no business
butchering mastering music anymore due to hearing loss, age-induced or otherwise.