I've seen switch arms fall off while playing and the ball hits the switch in a bad angle. It could also have fallen off while transporting/hauling but not that likely. I have had several screws and spacers just vanish while working on my pins, that arm probably lieing there somewhere too.

As you have a solder iron simply check the manual for the part number and replace it.

If you don't have a paper manual then check ipdb.org for a digital one.

Also, don't trust the diagnostics, for me it reports lamps broken but they work (probably cant handle leds). Use single switch and single light tests instead.

Forgot. Regarding that last switch not registrering. Use single switch test and manipulate the switch with your hand from below playfield, if the switch registers using hands but not a ball then just loosen two screws holding switch, adjust its angle and tighten screws again. Retest with a ball until it registers properly. If you cant get it to trigger by hand its probably broken and need replacement (assuming the wires arent loose).

If you are uncertain of switch condition, remove it from the game and use a multimeter in the continuity mode (should beep when its contact) and mesasure switch while pressing down.