If you are using plus, and want to play a custom level, you can change levels by pressing "t" to talk, then writing "/c " followed by the filename. For example, if I wanted to play "Happy Semiconductor CTF" and it's filename in my Jazz2 folder was semi.j2l, I would press "t" then write "/c semi". To rename your server, do the same thing, but use the command "/servername" instead of "/c".

Most other awesome things that can be done in a similar fashion. Read the manual (called plus-readme.txt, which should have come in the download with plus) to learn what each of the commands are and what they do.

Co-op will unfortunately not work correctly online, even with 1.24+. It will be playable, but people other than the host will not be able to lose health, and I think there are a few other major bugs. I don't really remember. There are other tools you can download to fix these bugs, although I personally haven't downloaded them and don't have any links to them.

The music should play just fine. If not, it's quite possibly because music, unlike levels and tilesets, is not usually downloaded when you join a game. It will only play if you already have the music the level is supposed to play. If you are the host though, and the level normally has music, it should play fine. I'm not sure what would prevent this, if that's the case.

I also don't know what would cause the darkened name from 1.24 servers. The dark color usually just means it's a different version, and if you're playing 1.24, there shouldn't be an issue.