I believe the actual 'coffin corner' was a one-way street, that real control loss was supposed to happen there with ANY change in Mach or IAS. It would require consumate skill to 'lower the nose' and fly out of it without increasing Mach no or reducing IAS. I suspect it was more a notional 'media' thing rather than a real one, but I'm sure someone will know better.
Ironically, for 'rubik', "Lots more power might help." - it was the converse. Too little power stopped people from getting up there as Lear pilots have discovered the hard way.