Thanks so much for your response and advice!
It is definitely NOT electrical issue
I kind of figured the same, but I'm relieved to hear you confirm it!
With the lever set in multi-exposure mode, it still only fires part of the time... With no back on the body, the shutter seems to engage every time at all speeds, so you're definitely right in diagnosing it as an issue between the back and the body! I've tried three different backs now so I'm getting more convinced its an issue with the mechanism within the body.
I have a fairly ugly film back that I wouldn't be opposed to attempting to file down the interlock pin as I've read that's merely what Bronica Technicians were doing when customers were experiencing similar issues. I don't know if that's true, but in my research it has seemed to remedy the situation for some people. Should I go for it?
There are two pins with collars on the film back, the one pictured below circled in red is the one I believe to be the Interlock Pin can you confirm?
Thanks again for your time and help, I really appreciate it! What a mystery!