what's a MU flight? MUC, from USA? or China Eastern? from somewhere

one can go higher/lower to take advantage of headwind/tailwind.

the speed of sound drops as altitude increases, and planes must fly slower at higher altitudes. this presumes they can fly near speed of sound at lower altitudes, also.

i do not know the exact numbers, but the window between mach1 and stall for say a 747 is very narrow at 35-40.000 feet.