Mixomax
I'm no expert but here's my understanding.
The lower limit of controlled flight is obviously the 1g stall speed.
The upper allowed speed limit is Mmo limited by buffers below the speed at which mach (high speed) buffet occurrs.
Stall speed increases with altitude and weight.
Vmo/Mmo is not effected by weight.
At altitudes above 30000' the 1g stall speed starts to increase significantly while the limiting mach number starts reducing the allowed IAS.
Where these two meet on a graph is 'coffin corner'.
At this point an aircraft can have only a few knots between low speed buffet (stalling due to increased Vs due altitude) and high speed buffet (where localised airflow around various parts of the aircraft, not just over the wings, first exceeds the speed of sound.
Clearly aerofoil shape will have some effect...I well remember dragging an F28 to 35000' after a MTOW departure and sitting there experiencing almost imperceptable low speed buffet until we burned off some fuel. When we banked into a turn the buffet increased because we were no longer in 1g flight but something in excess of that in a 25 degree angle of bank...our effective weight was increased therefore our stall speed increased.
Clearly a stall at high altitude with its attendant reduced aerodynamic damping (due thin air) is something that would really get your attention!
At the high speed end you can get mach tuck where the nose keeps going down and there's SFA you can do about it. This is caused by the center of lift moving aft when airflow around an aerofoil becomes transonic. The faster you go the worse it gets. This is also a function of aerofoil shape, as discovered by the Test Pilots in the High speed flight in the UK during the war. It's made worse by swept wings where the thicker inboard sections (forward of the tips) go transonic first.
Mach tuck is offset by Mach Trim which automatically changes the stab trim angle of incidence to trim more and more nose up as you approach Mmo. While conducting post C Check test flights on a Falcon Corporate jet I was required to exceed Mmo by a set amount and it took a fair bit of muscle to hold the control column forward against the mach trim as we slid through mmo and into high speed buffet in a full power dive from 37000'...twas very interesting excercise
Concorde, SR71 etc would not have any worries about high speed buffet as they are, most assuredly, designed to go at multiples of the speed of sound. Low speed buffet would be a different matter entirely. I don't know, but strongly suspect, that this is the reason for Concordes Mach 2.0 emergency descent in case of engine failure.
Hope this helps.
Chuck.