You will need the other model of the Holdup with the 1 1/4" Receiver. That's what the MiniDoMore Receiver is. There is, however, a 1 1/4" to 2" adapter part but MiniDoMore doesn't sell this...
Any device that goes into the Receiver will require the Pin. The way it mounts in the Clubman is behind the Bumper Cover, so yes, you access the Pin under the car. I am told that it's pretty easy to do. Hopefully I will find out in 3 weeks or so.