It seems like the game is “off loading” the enemy bullet to player collision detection to the HuC6280. This would have presented a very complicated challenge to would-be bootleggers back in the day

Having an invincible RoboCop would be a devious punishment for arcade operators bootlegging the game. One quarter would eat up a lot of time on the machine. It would be hard to make any money on it.

If you're not familiar, Hoffman is also a supremely talented musician as well (often inspired by and/or made on an Amiga).

His track "Eon" here is from a very famous and artistically beautiful demo. https://hoffman.bandcamp.com/album/the-cave-sessions

I can feel the passion. Great write up!

MAME emulates both CPUs?

So that means there are 2 ways to crack it. Emulate both CPUs, or rewrite the code from the sub CPU to work on the main CPU.

Sigh, one day I'll have the time to put my feet up and learn about debugging and assembly.

Personally thinking, I need to buy a breadboard and start tinkering.