Udev and systemd to merge

The way NetworkManager works is actually pretty simple. Unless a connection is marked "Shared", it is considered a potential upstream connection. Of all the upstream connections, the best one is marked the default gateway, with simple ordering (i.e. vpn wins over cable, cable wins over wireless…).
A connection marked as shared will get a DHCP server and uses NAT to route traffic over the default gateway. Therefore it is perfectly possible to share your 3g connection via ethernet to a bunch of computers.