I mean, he selected the company to sell his practice to, so whatever fallout from that is on him, not you.

Technically volume and gain and compression are separate things but they are related. Mixing process is generally where you adjust volume or gain on individual tracks and you can apply compression on individual tracks as well. Compression generally acts as “glue” during the mastering process to bring levels of individual tracks closers together by applying it to the master track. Very high level explanation.

I’m not familiar with musicians having much input on the mastering process. Could be their decision to not be involved. Before mastering though is mixing and musicians have a say in that, but not always.

Genres generally get compressed or limited differently and especially over the times music has become more compressed and louder.

Sorry I didn’t answer your questions exactly.