Well, I did find some PVC planks at Home Depot, and a friend with a table saw, so was able to replace my first window without using wood.
Now I have removed the back window. Again, the wood does not seem to be really rotten, but I had to pull some of the screws out with pliers, so I decided on a PVC frame again.
Removing ensolite from the wood frame is next to impossible. It's razor blade work, which can be dangerous - I'm a musician and I'd prefer to keep all my fingers. So I devised some tools and strategies for the job which worked very well.
First, I made a cutter by clamping a razor blade into the jaws of a pair of small locking pliers, aka vice grips. It worked very well...the blade did not shift, and it was much easier to cut straight along the wood under the ensolite than with a naked blade. Make sure that the vice grips have a flat surface...some of them only meet at the point.
To protect my fingers I put on some gloves I got at Costco. They are tight fitting, and have a little extra protection on the finger tips as a result of the material that allows them to be touch-screen sensitive. I went back to Costco for more pairs of gloves to give as gifts, but they were sold
Thanks my update to this thread.