Is the bolt head really split or just what perhaps an odd size? Or maybe rounded out by someone using the wrong tool? If I had to guess I'd say it is probably 6mm. But they could have used something else. Cinelli used 7mm for many years, an uncommon size.

As for the rear derailleur (which we usually just abbreviate to RD), most people don't take them apart except maybe take the mounting bolt out, maybe remove the pulley cage from the body, and certainly remove the pulleys to clean their bushings.

Mafac brakes, yes, well, they are one of the better stopping centerpull brakes. They do tend to squeal sometimes, and getting that to stop can be frustrating. Getting them centered, as with any cernterpull, is really just two things - make sure the caliper itself is reasonably centered when the cable is loose, and then when you attach the cable push the hanger across the straddle cable a tiny bit to even up the pull between the two arms. It pulls harder on the arm that it is closer to, which of course makes the opposite brake pull harder. Usually the hanger will stay put just from the friction.