[rm105]

Tappet plate being broken shouldn't effect semi/auto selection - the gun would shoot normally, but nothing would come out of the barrel (that's best case scenario - pieces of the tappet could jam the gearbox)

I'd guess you're onto the issue with that switch - its hard to tell for sure but it looks like the little plunger is not sitting right? If you have a multimeter, unplug the switch and verify if it's working correctly. You could also jump the pins on the connector to see if the gearbox works normally outside the gun.

Selector would be my next suspicion, test out of the gun so its easy to articulate. Sometimes gearbox alignment in the reciever can cause guns to act up like this.

This isn't a common platform so not a lot of us have hands on experience with this gun.