Ah thats interesting. If that is working without NDP proxy daemon, even with the NDP proxy sysctls activate, the kernel still needs static routes (as generated by the routed NIC type) to work. If its working without that or ndppd then it suggests your ISP are routing your /64 subnet directly to your host rather than expecting NDP resolution to take place. In this way your host is just doing the router part of the job and not needing to proxy NDP as well.
We have a Tutorials section in this forum (although ofcourse we would be happy for you to put a tutorial up on askubuntu too). If you post a tutorial and then we could link to it from our Tutorials section as well.