Keeping the Dark World Song after getting the Falcon

Yeah, it will play there, what you will have to do is use the level editor to change what music plays at the returner hideout, and possibly other places, I'm having trouble remembering. And you might have/want to hex edit some instances where that music plays, if there are any. I'm pretty sure after the Tentacle battle that song comes in.

Actually, that song is only used in... like... three instances, I believe. At the meeting, if you fight the Emperor's Special Forces while taking a break at the dinner meeting, and in the Figaro Castle basement BEFORE the tentacle battle (After the battle, it becomes the Figaro Castle theme).

you forgot entering the returners hideout as well.