Well, it finally dawned on me what I was doing wrong here. (Just in case anyone else makes my same mistake:)
I had the unRaid management ports set to 80 and 443 so naturally I had to set the http and https ports of NPM to something else. In my case, 180 and 1443. Since I had my router set up to forward WAN requests of 80/443 to 180/1443, all worked well from the outside. Inside the network however, when I access sabnzbd.mydomain.com it will implicitly use port 80/443 which of course just maps over to the unRaid mgmt ui, NOT npm of course. Doh!
So, I just changed the mgmt ports, and threw NPM onto 80/443 and all is well now.