I'm no expert in all of this, but I can attest to the fact that when I was using Comcast's internal "automatic" tunnel
all of the machines on my home network were being assigned and using 2002: addresses.
I have heard bad things about Comcast, but I can't believe they are that incompetent.
It is possible for an ISP to take one public IPv4 address and use that to deploy CGN and use the same IPv4 address for 6to4 such that they can deploy dual stack behind a single public IPv4 address. There is nothing wrong with doing that, and if it was done, the 6to4 part would scale to a larger number of customers than the CGN part would.
But it is only acceptable for an ISP to provide such a setup if they provide native IPv6 at the same time. And at that point the 6to4 gateway they deployed on that public IPv4 address would actually only handle packets in one direction. Packets from the 6to4 addresses to native IPv6 addresses would be transmitted as native IPv6 all the way without going through 6to4 gateway or 6to4 relay.
If that is what Comcast did, then it might be that some routers have a problem with having two different prefixes. If your router only picked up the 6to4 prefix and not the native prefix, then that could explain what you saw. This is just guessing, I don't even think this is the most likely explanation of what Comcast did.
It could also be that all Comcast has done was to provision 6to4 relays within their network without deploying any 6to4 gateway. In that case any customer with a public IPv4 address (which I suppose is still all of them) could run their own 6to4 gateway.
Functionally that would look very similar, except it wouldn't necessarily involve any native IPv6 addresses for the customers. If you setup your own 6to4 gateway, and Comcast have 6to4 relays for you, then packets from you will go through both 6to4 gateway and relay, but will still leave the Comcast network as native IPv6. You wouldn't rely on third party relays for packets in that direction, but you'd still rely on a third party relay for the return traffic.
If the later describes what Comcast had done, and you decided to use 6to4 as your only connectivity to the IPv6 backbone, then you made a mistake. You shouldn't have decided between using 6to4 or HE, you should have used both. It is possible your router isn't capable of doing both simultaneously. If your router cannot do both, then that sucks, and you should go with just HE, as that is overall a better choice than 6to4.
If you have two public IPv4 addresses from Comcast, then you can setup two different routers with 6to4 on one and an HE tunnel on the other. However be warned, that computers on the LAN might not understand which packets to send to which router, so it could end up being worse than just a single router.
A single router with both 6to4 and a configured tunnel on the same router is the most reliable IPv6 connectivity you can get, if you cannot get native IPv6 from your ISP.
I could dig up my old /etc/hosts file if that'd help.
It would be interesting to see. What I am most interested in from that is the IPv4 address, which is embedded in the 6to4 addresses.
I'm still scratching my head and wondering if that prefix is precisely why sites like Facebook just weren't working at all much.
If you are using a 6to4 address to contact facebook, then you are relying on whatever relay they are using for the return traffic. In both of the two possible setups Comcast would be in a position where they could guarantee a reliable connection in one direction. But they can do nothing about the return traffic.
It may be that facebook consider reliability to be a high priority and thus have deployed their own 6to4 relays for the return path, in which case both directions could be going through reliable relays. But it is also possible that facebook have no 6to4 relays, and thus are relying on third party relays. Then that would definitely explain why you experienced problems.
The only two networks I know of to have deployed 6to4 relays on a large scale is HE and Akamai. There may be others, which I don't know about. It is not always possible to find out whose 6to4 relay is being used.