I don't have any experience with NT Domains, but I think the mount password is stored in fstab (if you automount) and the Linux passwords are meaningless in your context. You need those if you connect back. Look into the smbfs package documentation for details.
There is no default password supplied by us. The password there was entered by you either at install time (new CD, starting 22.214.171.124) or on our website (old CD, 126.96.36.199 and older). The "pluto_*" users exist in /etc/password so that samba cand do mapping and I can delete and restore those entries at each boot and at other points without touching the system users. Samba doesn't use the "pluto_" prefix for its users when you connect to it.
There are 3 encrypted versions (MD5, samba and Linux) of the same password on the system, in the database, so using "passwd" will break thing, plus that it will be reverted at reboot
To change all 3 copies at once go to the Pluto Admin website on your core under "Wizard -> Basic Info -> Users" and click "Change password" on the left.
Also notice that the root password has nothing to do with anything of this. We don't touch that.