Another thought I had was that since the Proxmox shell has root access to the container, maybe I could create a sudo user that I could try instead. I followed this tutorial to create a sudo user and it seemed like it worked until I tried to execute the suggested sudo command "sudo ls -la /root". The result was "-bash: sudo: command not found".
So I went back to the Proxmox shell, "pct enter 103", "yum install sudo", gave permission, and sudo installed successfully. (why would I need to install "sudo"? Is this a security feature of containers? [newb thoughts])
Rebooted and went back to container console, logged into "sudo user", and successfully executed "sudo ls -la /root".
Still as "sudo user", I did "sudo passwd root", typed in "correct simple password", and again got "passwd: all authentication tokens updated successfully."
Logged out of "sudo user" and tried to login in a "root" with "correct simple password" and STILL no-go.
It almost seems like "root" is broken, but I also suspect maybe us newbs are missing a security feature/concept of containers, CentOS containers, or Proxmox-provided CentOS containers. I specify CentOS because opensuse-15.1-default_20190719_amd64 is another container I have running and it's root user has no problems.