I wonder ....... Are the active directory access and authentication issues being, at least partially, caused by the samba.conf file maintaining a security level of security=share instead of security=ADS? I believe it might be possible for the device to "look" like it's joined the directory by making all of the other Samba ADS items, but not changing its security level to ADS.
Also, is winbind running on the NAS? Has winbind been added to /etc/nsswitch.conf? Obviously, we can't answer these questions ourselves because of limited access to the NAS. But the questions are still valid.
Link to getting Samba to talk to active directory:
http://wiki.samba.org/index.php/Samba_&_Active_Directory