MoonbowMercury. I'd imagine that you would run into none of those troubles. I've never done it but I trust Minecraft to be decently coded enough so that it does not attempt to play music as another one is already running. I imagine that multi-layering sounds are only used for small-scale sound effects such as explosions and hitting and exp etc.
Generally, Minecraft chooses a song to play, depending on the situation (menu, end, frolicking, etc) then streams it to the game to avoid lag. Once the song has finished playing, the game waits a random amount of time before trying this again.
Events like the credits song would be a one-time thing so I doubt the music would loop. Similarly, I'm not sure what would happen if the music was longer than the credits.
Ah. That's good to know. Thanks, Mihiro.