When making music based levels, more people should make actual 'parts'. If you see my music level, I have a note player on an extended arm that keeps popping out from the left, playing the notes. Obviously you don't have to put them up like I have (actual musical notes on chains that you can adjust, lol), but you could make several different repeating parts play like this. You could even trigger the extensions by a simple 'macro' block and take that up in your more 'regular' music, which might make it easier to incorporate in your current way of working.
This should save you a fair amount of memory. In the meantime you could still have the player chase the melody. I've experimented with various triggers placing my melody notes throughout a regular level, either triggered by the player proximity or in the background by a moving bar (when I'm using actual musical notes)
The bunker seems terrible when you first get to it, but in the end it wasn't that hard. I got more frustrated with the AI bits later on, but I'm sure that when I get back to that again, it will eventually not be that hard either.
However, as a testament to the quality of user levels, I think some of them are starting to surpass what MM has done, which is awesome.