Mine is quite stable. I never have any question about wether the car is secure or not.
I can rotate my car with one hand.
I used large tubes where the upright and horz bar meet. I also greased it well before I put it together. I use a stop pin to prevent roation.
As far as CG. That is the whole point of the car bracket/arm attachment. It allows you to drop the body in relation to the piviot point so that you have the car neutrally balanced around the pivot point and it takes little effor to rotate the car.
I have my pivot point about 4.5 feet off the ground. It allows me to rotate the car about 300 degrees. The windshield A pillar hits the lower bar so I cant spin it totally upside down, however, I have not found a reason to do so. I prefer to have it a bit lower, if I raised it up on the main vertical supports I could spin it totally around, but have not had a need.