I fixed a Monte Carlo project A while back with the same problem 5v at the mpu measured low. After I replaced the headers on the 5v power supply new crimp pins and cleaned the 5v edge connector on the mpu with an eraser it came back to life. I typically add the pcb ground mods after I get the game functional my thinking is if its working the connections should be ok at that point then add ground mods for reliability. Its also way easier to remove the boards without having to mess with the spaghetti wires.

There are single and double sided traces on the daughter cards and the single sided ones can be problematic since you can't reflow it without removing it entirely and run a big risk of pulling the traces on the mpu. I normally yank them and replace them with ones from Fred Swemmer. If you are careful you can get in there with micro shears and just cut the card off and heat and pull the pins individually. I have fixed the originals and still had booting issues down the line.
Another quick thought if you have an eprom burner Rock only uses the one eprom on the daughter card so if you are suspecting issues with the prom 2 socket after you get the daughter card issue fixed burn the Rock eprom to test the board.

Read the roms with an eprom programmer and compare with rom files or romident. If they are corrupt or wrong burn the right roms and re test. The trick to working on system 80 machines is having more than one and lots of stockpiled boards.
Another thing to test the mpu for issues is using a prom 1 for Rock since Rock does not use prom 2.