For example, here is what will happen. Notice the axle is still parallel to the frame? Its pivoting on the hitch, yours will do the same, even with the single suspension pivot point in the middle...because of where the bags are mounted. It'll work great as a supsension design, do get me wrong, but it wont flex like you are thinking.
I pretty sure he's right because you have no solid mount on the hitch. In the early 60's there was a trailer you could mount with 2 attachment points to your vehicle. It had one wheel that had spring loaded suspension and pivoted also. The trailer would lean with the tow vehicle as needed. The only way you'll get articulation from the trailer is if has 2 axles one at the front that can steer and one at the back. The front will climb or droop and the rear will do the opposite depending on the terrain. The trailer has no need to articulate when it is doing so at the point of attachment!