To conclude this thread, I was able to fix the 2 buttons which had the plastic pins snapped off by following the advice from Malte Rogacki. I drilled a small hole where the plastic pin was, and inserted a small metal pin in its place. So thanks again Malte for that great idea.
I was also able to use the Alps SKHCBEA010 switches as replacements even though they have a larger hole in the top of the actuator than the original switches, and by placing the Matrix 6R so it was sitting on its back panel, I placed the buttons on the switches, then carefully put the front panel pcb back inside the Matrix, using gravity to hold the buttons in place on the switches. Once the front panel pcb was screwed back in, all the buttons are held in place supported by the round cutouts in the metal frame of the case, and the front panel cover. After testing for several hours and pressing all the controls, the Matrix 6R works perfectly once again.
Hope that helps someone else sometime :)