The ISP peeked at a config somewhere on their network and found that to be responsible for my ails. On seeing the e-mail, I noticed my network was correctly self-healed and connected via that gateway!

I rejoiced, tested a few things, and then set to move on again.

I re-enabled IPv6, and suddenly my WAN router started failing. Turning it off again set the network right. There's some poking to do on that, but IPv6 can wait.

I've got some researching and tweaking to do to get the multi-home network to behave on my server. Sadly, almost everything I've found so far either assumes one of the multi-home networks is private, or that any Internet traffic should be able to go out the single default gateway.

I'm going to make the server work next, then get the IPv6 working again. It worked before...it shall again, but later!