After 6 days of not daring to touch my router because of oncall I finally configured it now. Most surprising lessons:
1. Linux' accept_ra is a tristate: 0=off, 1=on iff forwarding is off, 2=really on
2. Debian's dhclient will remove all addresses from an interface it's about to do dhcp on. So if you try to get a prefix delegation it will in fact kill your Internet connection.
3. Debian's dhclient does not execute user scripts, it sources them. So if you use exec or exit you're not getting any addresses at all.
4. dhcpd can not assign static IPv6 addresses based on the MAC address of the client and can not push an IPv6 default gateway. I'm sure that looked like a good idea at the time.+Norman Rasmussen