Capacitive crosstalk switching frequency vs rise/fall time
Let's assume we have 2 square waves, one of 1MHz and one of 10MHz,
  both with rise and fall times of 10ns.


And this is contradicted by this: -


  If we decompose the square wave into harmonics, the 3rd and 5th
  harmonics are also 1/3 and 1/5 in amplitude and so on.


For perfect square waves that would be true but you have told us that you are using waveforms with real numerical rise and fall times of 10 ns and, as you approach frequencies like 100 MHz the results won't be as clear cut.

But generally the individual harmonics of the 1 MHz waveform at (say) 49 MHz will be much lower than the 5th harmonic of 10 MHz (50 MHz) but, harmonics are occuring much more often with the 1 MHz waveform (i.e. 47 MHz, 49 MHz, 51 MHz etc.) and this also needs to be factored in.

In short, your conclusions are faulty because you are not considering the bigger picture in the frequency domain. Analysing the crosstalk from the perspective of rise and fall times is the clear-cut method as you have demonstrated.