A broken diode would definitely cause your outhole switch to not register. Original part is a 1N4148. 1N4004 will work too. And based on the curve of the switch plate, it looks like the screws need to be tightened.

Individual sockets can absolutely be replaced. I like to use solder wick to remove solder from the base where the bare bus wire connects. Then just desolder the ground wire, unscrew, replace. Bally sockets from this era stink!

Hadn't seen that Kiss lamp board before - very cool.

Pinball Life also sells single lug lamp sockets that work great for Bally insert lamps, but obviously these won't do much to help your metal plate situation.

http://www.pinballlife.com/index.php?p=product&id=992

Personally, I like the cleaner look of the single lug sockets and last time I checked with PBR they only had two lug sockets in an insert depth. Of course, either will work.