Nobody has responded to this, but I’m just reporting back to say the problem has resolved itself. Beginning Thursday, November 30, I disabled IPv6 on the Netgear and left it that way in order to let Windows Update run normally. Beginning Wednesday, December 6 I re-enabled IPv6 in order to see if the problem was still present. Windows Update has worked normally ever since.

I’m pretty sure this was not a home network issue, even though it might sound like one given the workarounds I reported. I believe this because neither the appearance or disappearance of the issue corresponds to home network changes. The problem seems to be rooted in some external factor. My network experiments may have provided a workaround simply because they resulted in a different IP addresses. (i.e. IPv6 vs IPv4 when toggling IPv6 on and off, and two different IPv6 addresses when switching between the Netgear and the PACE). The different IPs may have in turn affected the server choice or the route.

I believe this is just a reminder of how fundamentally temperamental and unreliable the internet really is. It’s next to impossible to diagnose a failure or determine responsibility. Best not to become too dependent.