IMHO - I've just learnt that - all aircraft have a coffin corner, it's just that some can't get there for lack of power or equally lack of a limiting Mach. Absolute coffin corner is where the stick shaker is on and you are at MNE. Practical coffin corner is where you are on the low speed buffet boundary, about 1.2Vs -at that height - and on the high speed buffet boundary -usually MMO. Many transport aircraft can't reach coffin corner fully loaded because no one is going to put in excessive engine power above that needed for economic cruise. Old square overpowered military jets did reach coffin corner.
Stall IAS does increase with height. The constant speed, weight for weight in level flight, is EAS, and for the same EAS IAS gets higher and higher as compressibility comes in.
Likewise, MMO gets lower as alpha is going up at the lower EAS at height, and with increasing alpha the shocks come earlier. The best picture I have seen of this is the 737 set of buffetboundary charts.