A source of confusion over the AoA-Mcrit discussion is grouped around the fact that AoA's for typical jets in cruise flight are fairly small, a degree or two, and variation with altitude is minor as well. The focus goes to critical mach number while holding AoA constant. You can even use an AoA of zero if you want. That's smart because a meaningful comparison is aided by removing the variables to allow direct comparisons. AoA, dynamic pressure, sometimes even speed is eliminated. It's sort of like trying to find a short circuit in your house, you need to figure out which circuits are not affected before you can determine those that are.