I mean a little bit of play is required for the rear hinge (for folding), but not that much as you wrote above.
Did you have a look at the plastic (?) part 100-07?
Maybe this part (and the corresponding holes of the bottom tube) widened out by the time?
However, I’m still very interested in the wheel stiffness, today I’ve prepared some parts for a first test and measuring results will follow asap.