dd- a simple answer!
'Coffin corner' is where by going a few knots slower you stall, and a few knots faster you exceed your limiting mach number and encounter severe 'compressibility' problems making it virtually impossible to descend without losing control. Concorde was flown at Mach 2 in the cruise (and the SR 71 a 'little bit' faster), so you would have to be VERY high to be anywhere near stall speed, even for those machines, at that Mach. Perhaps some fundi could tell us both the stall IAS of Concorde and the altitude at which this is equivalent to Mach2?
I recall an X-15 getting into a spin at Mach 7 but I guess that was a foot or two higher than Concorde
