I am nothing if not tenacious when it comes to pursuing an idea for a solution, but I've finally thrown in the towel in my effort to try to convert my computer's ethernet port into a USB port. I've tried every possible angle. An adapter which I thought would do the trick is actually just a physical adapter, with no signal conversion. It's intended to be used in pairs to allow a USB cable to be used in place of an ethernet cable - one would be put on each end of a USB cable to effectively turn the USB cable into an ethernet cable. So the signals remain ethernet signals. (There are adapters to turn USB ports into ethernet ports, and they have an electronic module to convert the signals. But there is nothing to go the other way, unless you want to make it yourself.)
But I think it's interesting that there is no way to get a wired router with a USB WAN port. They're all wireless, advertised as good ways to securely share your expensive wireless internet connection - trust us.