I've spent more hours but haven't made any progress. Here's where I'm stuck. In a proxy router's client's browser, if I type "http://fiddlersystem" I see the request coming into the gateway/Fiddler(proxy set to port 80) system. If I type a website, e.g. "http://httpbin.org", I don't see anything coming into Fiddler, Charles, or any other listener however the proper response is received. So it seems that Windows is not passing the http request through to Fiddler.
Note that the above works perfectly using OS/X with mitmproxy.