Bob,
"Windows 8-10 have driver signing / enforcement turned up by default." If that's the problem, then why doesn't Arduino just sign the drivers?
"If both kits are official (sounds like they are)." Mine is.
"The drivers would normally install during the IDE installation." They didn't.
"If this was not the case you can go to the drivers folder in the Arduino installation and install from there." They won't install.
"I generally recommend doing this as ADMINISTRATOR or with driver enforcement turned OFF." As far as I can tell, you can't run Device Manager as Administrator. Driver Enforcement won't turn off because of security policy. Again, why can't Arduino sign the drivers?
"There are multiple posts about this type of topic . . ." Before I originally posted I did search on the topic. The only thing I found out was that this has been an ongoing problem since at least 2015. Nobody has posted a definitive fix for it.
Does anybody know if any other board manufacturers provide signed drivers? Might have to go that route, unless I just scrap the whole Arduino idea.