This isn't really related to this project, but we did figure out the root cause of the router instability. It's the fault of our Samsung smart TV. It's apparently using the hostname "localhost" in its DHCP requests. That causes a bunch of logging and other complaints from the DHCP server built in to the router's dnsmasq instance. What then causes that to result in the interfaces wedging is unclear, but giving the TV a static address made the problem go away.

EDIT: Well, it turns that the jury has not yet entirely returned a verdict yet...