"dns forwarding" now looks like this:
forwarding { cache-size 1000 listen-on eth0 options enable-ra options dhcp-range=::,constructor:eth0,ra-stateless options dhcp-range=::,constructor:eth0.102,ra-statelss options dhcp-range=::,constructor:eth2,ra-stateless options dhcp-option=option6:dns-server,[2001:4860:4860::8888],[2001:4860:4860::8844] }
and all the previously-mentioned settings were removed, as suggested.
Rebooted router for good measure.
IPv6 working fine on all clients... but DNSv6 servers still don't show up in any of the clients.
Functionality seems identical to the "service dhcpv6-stateless" configuration tested just prior. This approach is fewer lines in the config file, but requires dnsmasq to be running.
Then I noticed that I have dnsmasq disabled for dhcp-server. Could that be affecting things? It didn't like it when I enabled it in the config file. No route to server for anything on the clients (IPv4 and IPv6).
Back to running the previously mentioned settings (service dhcpv6-stateless).