Here's how I think I would connect everything. The mosfet only has 3 pins, so I'm not really sure how that breakout board exposes 7 pins. If you are not daisy chaining lights, then you can ignore the connector on the right.
Your description of connections to the mosfet board seem reasonable. I didn't find a schematic for that board, so I'm not 100% sure what is connected where. I'm almost certain you are correct though.
You are also correct about using a single light port on the goby hat. That's exactly how I have it in my test picture, and I'll have mine connected to the Light2 port. The following picutre also shows how I spliced into the mains voltage. Just be sure to do this with the fixture unplugged and secure and cover any mains wiring.