Issue with circuit design 10x probe
The thing you are missing is the fact that the cable on a scope probe is NOT ordinary low-loss coax, mainly because it would be difficult to make a usable piece of coax that has a Z0 (characteristic impedance) of 1 M&Omega; that would match the input impedance of the scope.

Instead, scope probe cable is a special lossy cable that minimizes the effects of reflections over its length. It also helps mitigate the effects of the cable's own capacitance.

If you want to use regular coax, you'll need to match the outputs of your attenuators to the impedance of the cable (a buffer amplifier is the most direct approach), and use the 50 &Omega; terminations on the scope's inputs.