Interesting analysis. Based on the OPs description of flex & shifting, I assumed that there must be something wrong (i.e. broken) with the frame. When I see the photos, that is a long, short frame. All else being equal, the torsional rigidity of the frame is proportional to the cube of the height. With no lateral tube, there is a lot of twist on that frame with little height to resist it. Kind of like making a convertible car by cutting the roof off of a regular car without making any provisions to provide stiffness beneath the floor.
If LandShark took the frame back, will they be able to fix the flex with some of the modifications proposed by mtseymour or is it just a lesson learned?