Sorry for not responding earlier, but I managed to get it all working in the end. What I didn't realize was that the container paths specified referenced root, i.e. /downloads /incomplete-downloads, where as SABnzbd was referencing them locally from the /config container path i.e. downloads = /home/docker/.config/sabnzbd/downloads and incomplete-downloads = /home/docker/.config/sabnzbd/incomplete-downloads. As soon as I changed the paths in the SAB config (not the docker config) to use /downloads and /incomplete-downloads everything started working.
Hooooooooly this helped me out so much. I couldn’t figure out for the life of me where the folders got mapped to within the containers themselves. This makes so much sense. Thanks!