Well it appears Zune software is now, perhaps forever, broken. Today when I tried to login to Zune 4.8. When I try to sign in, I get the message "A secure channel couldn't be established. The server certificate name doesn't match the server address." Without being able to sign in, I can't download subscription music in the marketplace, even with the registry change I already described. I can however sync and even add subscription music to at least one of my Zune players (one authorized as a guest!). Not sure how long that will last, but without the ability to download new music, it's not too useful. We'll see if this ability last; I expect it might last until February 2017 when my subscription expires.
Not much information on this error on the web. If anyone stumbles across anything that could help, please post here, the haunts of the last living Zune user.
UPDATE: Apparently it was a temporary network problem on their side with whatever "server certificate/address" means, or some router or network problem on my side. I could Sign In today and resume downloading DRM files, while the service lasts.