Re: frozen shower diverter value
If you can't get the valve apart for cleaning and repair, then install a new tub spout with a built in diverter. It won't be quite as good as your current valve set up, but it will work and $10 is better than several hundred or thousand dollars to change out the valve.
BTW, soaking the valve stem with WD-40 isn't going to do anything if the stem nut and washer are still in place. If you can remove the stem nut and washer then you might be able to use a penetrating spray to loosen the valve stem.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!