Current ending to this story is:

- I have the system fully working again. For now. Crossing my fingers it stays that way.

- I spent some time talking to a nice lady at the local Cisco office and going over my options. She was convinced that there's no reason for anyone at Comcast to be deliberately bricking my modems. But in terms of which modems I can get, there's pretty much no options other than the ones I've already been trying.

- As a last-ditch hail-mary pass, I ordered (and finally actually got) a used SBC D3GNV modem from a different internet seller. This is the model I'd originally wanted to get from the other internet seller, the one where the other seller sent me the wrong one, the Cisco that turned out to be a stolen Comcast rental modem still in Comcast's inventory.

- Like the Cisco before it, this used SMC D3GNV one is an XFinity-branded rental modem that they don't normally sell retail.

- However, I got lucky: this one wasn't a stolen modem still in Comcast's inventory. Supposedly this one was legally sold out of Comcast's inventory and was removed from Comcast's database the correct way. Thus it was OK for Comcast to activate it, and I had no problems like I'd had with the Cisco.

- It activated fine and it's working fine (so far). So everything is back to normal for now.

- Its firmware, when looking at it through the web browser, looks nearly identical to the last Arris modem that I'd tried which had died after a few months. This worries me. However this firmware is working better than the last one: At least this one saves its settings when it's rebooted. Crossing my fingers that this means the firmware is actually different under the hood, and that this one won't die after a few months.

- The thing is, this is literally the last modem I can try. I've run myself out of options. I had tried every other kind of VOIP-capable modem that was on the Comcast compatibility list. All the Arris units die after a few months, the Cisco unit isn't allowed to be activated as a customer-owned retail modem, and every other possible modem (a couple of Technicolor brand units are all that's left) are in the same boat as the Cisco unit (according to the gal at the comcast office). The gal at the Comcast office seemed to indicate that the Cisco unit might one day get sold at retail, but not yet.

So this SMC modem better last forever!

If this one doesn't last me forever, then I have to go another route for the burglar alarm. One weird option I have available to me is to double down on Comcast: They offer a package where, if I were to buy my burglar alarm through them, then it would use the internet to call the cops and have a cellular backup if the internet is down. I could get rid of the landline entirely, and just use their alarm system, and still get their "triple play discount" (where one of the three things is now the alarm instead of the landline). The math works out so that I would end up paying slightly less overall (like only a couple bucks less total) for a TV+Internet+Burglar package via Comcast than I am currently paying for TV+Internet+Landline via Comcast plus Burglar via ADT.

Though it's tempting to make that switch from a cost perspective, I really have problems with Comcast's customer service, and a burglar alarm isn't a place I want to be cutting corners on customer service. Also, Comcast's system would not be able to make use of my existing keypads and sensors, and would have a weird thing where I have to use a smartphone app to manage the alarm. Ew. So Comcast's system is not looking attractive to me at the moment.

At the same time though, ADT keeps raising their fees a few bucks every year. That bugs me. So maybe I'll call their customer retention department and see if I can get them to quit it.

Oh, and a final note: I was on the customer service line with Comcast and even though I changed the password on the modem's firmware, Comcast still has a wide open back door into the modem that they can get into and fiddle with its settings. From the internet. The guy could see all my menus and settings in the modem's firmware. Ew. So my paranoia about someone bricking the modem isn't unfounded.