Fortunately, I did find a solution. I was surprised that I also did not find anything in the way of similar experience among the many tech problems people have because people with this problem must now number it the (tens of?) thousands. On another forum, however, a smart/pragmatic commenter wondered why I didn't just install a different server. Apparently the Windows server is a component of WMP, which Microsoft lost interest in quite some time ago. The software is simply outdated, so it's likely an OS update superceded its compatibility. That's as far as I went in tracking it down - what's the point of going further if you simply know it's not going to work any longer?
Painfullly comic how you went through all the pointless agony that I did trying to test out different theories.
So I installed Serviio on the PC and linked my video folder to it. My tv picked it up and it works like a charm. I just ignore the PC icon on the TV source selector and click on Serviio. From there it works the same as the PC server did by working your way through the folder selections to the folder you want.
Let me know how you get on.