Does the slotted screw come out or does it simply just spin? Also the gear spinning freely is either a sheared or missing woodruff key. Ace hardware sells replacement ones, I'm not sure of the exact size as I just compared the ones in the bins to one I had. They are only 50-60c so you could easily get 5 different sizes to make sure you got the right one. I was quite surprised at how perfectly it fit, but it would make sense they would use an off the shelf key.
The pad damage looks like the clutch plate was dragging/not fully releasing. I've battled that issue for months now and finally fixed it just today by going to Ace hardware and finding a metal dowel that was almost 1/4in longer. For the first time ever my clutch will hold tight enough not to slip under peak power and will simultaneously be able to completely release with 0 dragging at a stop. The dowel was all of 56c.