Ethernet is the only way to communicate between computer and mcGateway.

To be specific we communicate over UDP and TCP/IP. The UDP is required because mc-Studio needs to find gateways. After a connection is made everything goes over TCP/IP. Because UDP is involved you can only connect mc-Studio over the LAN to a gateway. We will add MQTT to mc-Studio in the future. This means that mc-Studio can connect to any gateway in the world if that gateway has a connection to the same broker.

I have had success simply publishing/subscribing via the mcGateway. No MQTT broker involved.

You can publish to any device that is connected to the same gateway and subscribed to the topic. If you want to publish to a device that is connected to another gateway you MUST use MQTT. The MQTT broker can be in the LAN or in the cloud.