In the end, I moved the PCE-AC68 to a Fedora-based linux box and bought a new PCE-AC88 for my Windows box. It installed properly and is working. So my repurposed PCE-AC68 is working fine, and my new PCE-AC88 is also working fine. I realize that's not a valid solution for everyone, but for those of us who have to have our systems functional immediately, that one will work. What's nice about Linux is that once hardware is supported, it seems to be supported forever without undue breakage -- a philosophy sadly lacking from Microsoft. Microsoft might take a page from IBM's backward - compatibility philosophy, and assure that once hardware works under Windows, Windows won't break that hardware. That's why IBM implemented the 7090 microcode in S360 -- they allowed their customers to run really ancient operating systems and business software on modern hardware without problems.
Best wishes to those plagued with this problem.