When I had to upgrade my modem a few years ago I ran into a similar issue. I ended up going to a Comcast office and picking one up myself. Second issue I ran into was provisioning it. I tried to do it over the phone and things progressively went from bad to worse. Finally when a tech was sent to my house he suggested in the future I never try to provision over the phone it usually causes more issues then it solves. He fixed the problem in 5 minutes. Turned out they had the wrong cable location information on their computer. He also told me since it is a Comcast issue I should never pay a fee for a tech to come out.
You said "FWIW, this was a totally ridiculous statement made by a single person...." I am curious as to why you said that. One would think Comcast techs are knowledgeable. I am not disputing what you said just wondering why you said it for FYI for myself.
Yes, some are well trained and knowledgable but from what we've seen here and at other tech forums for years is that many if not most are not very well trained at all. Modem's have been being provisioned by phone from the very beginning. It is not the use of the phone method that is a problem, it is the person behind the phone who needs to enter the complete and accurate data in to the database of their provisioning system that can be.
This, and in an effort to reduce their phone support costs is why the came out with their automated self provisioning modem registration page (the Walled Garden).
The forums are mainly a customer to customer help forum. It is not a direct line to the Comcast help desk. No Billing Reps or Customer Service Reps read the forums at all, most are not even aware of the existence of the forums.
Please edit your reply and remove your personal information. The forums are public.
You can try the link below to have a modem sent to you. I have heard it takes a while to receive this way. If you have a local office nearby you can exchange your present modem there for a new one.