First, I have authenticated many computers on this network. Yes, root has had a password in smbpasswd since the start, many years ago (more on this in a moment, though).
As to firewalls, turned off (behind my hardware firewall) temporarily until the machine is set up. But I am not open to the outside owrld, either at this point.
All of the other things are set correctly - checked, double cehcked, and dtriple checked over the course of 2 days. And while I have been frustrated, its only a computer! But thanks for worrying!
Back to root smbpasswd, though.
Problem was solved this am. I had rechecked my smbpasswd, as I realized this was an user authentication issue. Sure enough, it was in smbpasswd. But for some reason I thought maybe, just maybe it got corrupted...so I redid the smbpasswd for root.
Sure enough, it worked. I can only think that I might have corrupted it while deleting a user in the past, and this was the first authentication I was doing since then. So, while I had root and a password in smbpasswd, the passwd was incorrect.
Up and running now, everything going great!
Thanks for the help & suggestions.