In the future, back out your screws slowly. If you notice the insert backing out, reach between the ski top and the plate with a needle nose pliers and grab the insert, then try backing out the screw. I've had this happen before with a tricky insert and that method worked for me to get it loose. Once I had the plate off, I took out the insert, put some JB weld on it, build an "insert setter" tool which is what @bojans describes above, and put it back in place. It's still holding up well for its new owner.