Coffin corner is certainly the common name given to the top left hand corner of the flight envelope (top being high and left being slow)
All aircraft become unflyable at some point due to low speed whether we call it a stall or whatever we think the airflow is doing does not really matter and those details will vary from type to type. Many subsonic aeroplanes (but not all) go out of control at some high mach number or other (again it does not matter why) Coffin corner is where the low speed limit and the high mach limit are coincident.
For some modern and high speed aircraft they may not actually go out of control at high mach but may have a design mach limit imposed. A few have neither and so do not have a coffin corner.
When Walter Gibb set the world altitude record at 65,889ft in a B2 Canberra with Olympus engines on 29 August 1955 you had better believe he got right into the corner….
At max Mach speed what happens? Is it a compression stall
rotornut
There are about as many varied reasons for an aircraft to go out of contol due to mach number effects as there are for loss of control at low speed.
In both cases you will no longer be able to properly control one or more of the following: pitch attitude, roll attitude, yaw attitude or height. In a few cases this may be because one of your controls has hit the stops in many others the handling just becomes too erratic (like a wing drops or there is an uncontollable pitch trim change)
About the only generalisation that applies to the high mach end is that shock waves are spoiling the way the aircraft is flying. (They could stop a control from working, they could cause asymmetric lift or drag, they might spoil wing lift they could cause pitch up and so on).
JF