I have an interface called broadband-ipv6 which is functioning as a tunnel endpoint, with DNS resolving properly; so far so good. When an ICMP ping arrives through this interface it's not clear to me how it gets routed out my dev eth2, to distribute the allocated /64 to the other ipv6 devices already on the local lan.

ip -6 route ls shows the tunnel endpoint address space dev broadband-ipv6
and the separately allocated /64 going to dev eth2

I can ping the separately allocated /64 manually on any device within the local lan. What command or statement will route between the two interfaces / address spaces?

Are you using the separate statically routed /64 for the LAN and not the tunnel's /64? Can you paste your interface and routing configurations?