Are you using the libnat64 library? If you are, I recommend you switch to the nat64d package. libnat64 breaks dnsmasq's dhcp server. Additionally, using the nat64d package means that regular ipv4 traffic will work on your laptop.

You are getting a link-local address because dhcp clients will configure that first to request the address from the dhcp server. For my scripts, DNS settings come from DHCP. IPv6 addresses come from radvd. So it sounds like you're not getting any DHCP responses at all, only radvd announcements.

I also cleaned up the scripts for connection sharing:
http://code.google.com/p/n900ipv6/source/browse/share

Following the README in the usb directory will set the n900 up to automatically share when you're connected via 3g/2g and plug it into a usb device.