Scotty, I have been working on this for awhile thus my late reply. Her is what I have come up with.
I looked at the flexible rigidity problem you are having and I have found that the real problem is the stagnate movement of the inner out side diameter of the wheels. This causes a gyro effect that is stopping the normally strong space frame we installed which is strong, instead it flexes to an extend that the high speed low area of the arm rest extend and warp the chassis. I have now developed a new chassis made from fibres from a Lama’s coat, cross hatched with SSG (Special Strength Gamma) carbon fibre.

Also looked at your straight-line corner speed. I put it in a wind tunnel and found the problem. To much toe in reduced the corner speed and too little made it too unstable and hard to control in a shopping centre. I have come up with the solution, it did not have enough toe up. Yes toe up, ok let me explain.

Toe up, when Nana was sitting in the wheel chair her feet were level with the ground. This increased the ground effects of the chair at high speed but the balance was all wrong. Her feet were causing too much upper level drag and not enough wind flow was passing below the chassis. With Toe Up it has improved the ground effects and reduced the frontal area that passes over the top half of the aero package. Also I developed lowered pods for the axle.

Dr Lowtech