I went through the process of creating an OpenBSD-based gateway for my home network (IPv4 and IPv6). Learned a lot and documented my setup in a blog post[0]. Maybe it can help troubleshoot your IPv6 setup. Pay special attention to these sections: (a) cnmac0; (b) dhcp6c; (c) The "Wrong" Config.
[0] http://codeghar.com/blog/openbsd-network-gateway-on-edgerouter-lite.html
