I don't believe that the areas were designed to flex (the flex is likely due to the cost effectiveness of spot welding). However, the flex would have been present during testing. You are correct that the stress would be transferred elsewhere, and strengthening the shell likely pushes the car into untested, unknown territory. The same risk would exist with the installation The value of seam welding cars of a similar category to mine is supported by its continued use by premium restomod companies like Retropower, who state that seam welding of window and door apertures greatly improves rigidty of the cars they do, and Alfaholics. Granted, these shops likely understand the additional strengthening required to mitigate any shift in stress.