What happens if you comment out *both* "auto eth0" and "iface eth0"?
What happens if you *un*comment *both* "auto eth0" and "iface eth0" with "managed=false" in the "[ifupdown]" section of /etc/NetworkManager/NetworkManager.conf?
What happens if you uncomment both "auto eth0" and "iface eth0" with "managed=true" in the "[ifupdown]" section of /etc/NetworkManager/NetworkManager.conf?
What happens if you comment out *both* "auto eth0" and "iface eth0"?
What happens if you *un*comment *both* "auto eth0" and "iface eth0" with "managed=false" in the "[ifupdown]" section of /etc/NetworkMan ager/NetworkMan ager.conf?
What happens if you uncomment both "auto eth0" and "iface eth0" with "managed=true" in the "[ifupdown]" section of /etc/NetworkMan ager/NetworkMan ager.conf?