have mentioned speedplays, in particular the frogs because I want a walkable shoe/cleat, but the guys there don't seem overly optimistic or eager for me to try them. I've spent hours googling and doing other research and many people claim the free float can be wonderful for people with sensitive knees like mine. Should I insist on trying them? I am ready to give up on the SPD's altogether. Thanks for your help and encouragement....
I had knee problems. After getting a new fit on my bike... and switching to Speedplays, my knee problems were solved. It took about a month of spinning and pedaling slow... but I fixed the problem.

I SWEAR on the Speedplays... I know that if I hadn't switched... I would have fought my pain longer than necessary.

It's also really nice to have a pedal that you don't have to keep going back to get "fitted" for. The float is great, in my opinion.

Now, other than the pedals and proper alignment there... are you SPINNING? What is your cadence when you ride. If you mash the pedals and don't spin, that can cause issues.