Rainbow Dash and Derpy planes

#1
like you said , the mach cone appear when approaching the speed of sound, and considering some fluids mechanics and thermodynamics concepts we can conclude that Rainbow Dash's speed is between Mach 2.5 and Mach 3.2, meaning that her acceleration is only 7g,so not harmful for experienced racers

#2
to be able to stand on clouds, a Pegasus mass should be around 3x10^-8kg, that explain why Applejack was able to launch Rainbow Dash so high and Fluttershy can land on butterflies.

#3
the rainbow making part of the sonic rainboom might be caused by the fact that Dash's able to ionise the air at high speed and give energy to it by friction. the rainbow is simply made by the atoms in the air releasing that energy by producing light. but that is only the most likely theory i have.