I found the question you wrote several months ago. If you still have the question, I try to give a few answers. Maybe someone else will read this and solve an issue...
Short answers for the questions:
1. # diagnose ipv6 address list
you get back fe80... addresses and configured once
2. no gateway in dhcp6, delivered by RA from router
Now a few more details:
In IPv6 you must tell the client with router advertisement (RA) flags to ask for the DHCPv6 information.
There is the "other" flag for stateless DHCPv6 (only information line DNS-Server, NTP Server and so on) and the "managed" flag for stateful DHCPv6 which includes the IPv6 address for the client.
Example to configure: (FortiOS 6.0.13)
config system interface
set ip6-send-adv enable # send of RA in general for this interface
set ip6-manage-flag enable # set managed flag = client get address from DHCPv6 server
set ip6-other-flag enable # set other flag = client gets for example DNS-server using DHCPv6
# don't forget to configure prefix list for RAs and activate sending RAs
Next step is to configure the DHCPv6 service for this interface:
config system dhcp6 server
edit 0 #use next free number
set lease-time 14400
set domain "example.com"
set subnet 2001:db8:1234:abcd::/64
set interface "port4"
set option1 56 '0001001020010db828000000000000000ac30123' # option for NTPv6 server IP address
# NTP server IPv6 address in this example is 2001:db8:2800::ac:123
set option2 56 '00030010013304706f6f6c036e7470036f726700' # option for NTPv6 server name
# NTP server name is 3.pool.ntp.org
set start-ip 2001:db8:1234:abcd::c:1
set end-ip 2001:db8:1234:abcd::c:ff
set dns-server1 2001:db8:1234:a53::53
set dns-server2 2001:db8:1234:b53::53
In the DHCPv6 configuration in NO default router/gateway. This information must the client take from RA packets. This is different in DHCPv4 :)
I hope this helps someone to configure IPv6.