Hey, OP Again!
Thanks all for your suggestions! I KNOW the replacement suggestion is probably the best and most long-lasting of all, and I will keep the instructions handy in my gmail folder. But this IS a 1970's era fiberglass tub and I am afraid if Imess around too much, I may inadvertently crack/stress the fiberglass.
And, since I already had to pull out one other showerstall from that era in the second bathroom because of that very problem (cracked fiberglass) about 9 months ago, not ready to chance that quite yet.
Anyway, I put a generous amount of Duralube Silicone spray on it yesterday, worked the diverter back and forth several times and finally left it in the "pushed in" position for about six hours. It now works very smoothly for the time being. Time will tell.
And, yes, the mineral magic does work very slowly, but I am using it on a toilet that has had "disgusting" hard water deposits for the past
15 years that we tried to remove with any number of items -- with no luck. So, far, using the Mineral Magic since about May, I have managed to get about 75 percent of the deposits off. Once a week treatment. I figure I will go through the entire gallon about the time it all goes away!