Well as far as the DHCP relay is concerned, one is the current limit. Not sure why they chose to do this but i am hoping we see the ability to use more than one in a future firmware release.
As for the Native vlan, On the 62XX series i have worked on when a port is put into a true Trunk mode(not general or access) then there is no native vlan as it only supports tagged packets. Now if your in general mode then you can have the tagged traffic and untagged traffic which would required that the PVID be set for the port or trunk.
hope this helps