Zipper I think you are getting to hung up on numbers, as far as I know the first thing to be affected by compressibility is the instrument that measures the planes speed, indicated air speeds of mach 0.92 are a bit of a joke with a pitot tube.
When a planes prop drops off or its wing wrinkles after exceeding maximum permitted speeds I think this speeds were calculated quite well.
From my experience of civil (pipeline) engineering I draw the following conclusions.
A maximum mach number isnt the value that a plane will fall apart or lose control it is the value below which the plane will stay together and have some control, exceed it at your risk.
The values resulting from calculations are useful to make sure there are no serious flaws in design with the technology known at the time, to work on improvements to the design AND to set operating limits for the planes in service. It is all very well to talk about a Spitfire reaching mach 0.92 (or whatever) and landing without a propeller no one ever mentions the many planes under test or in service that broke up or simply went straight into the ground. High speed dives in WW2 aircraft are extremely dangerous in both combat and test scenarios, it is a leap of faith as to whether the plane holds together and pulls out of the dive.
A maximum or minimum limit in my work experience is based on the materials specified yield strength and specified minimum wall thickness. In practice the material is always at or above the minimum yield and the thickness is at or above the minimum. The calculation of a maximum mach number will therefore be a number that sets the limit of what is guaranteed or controlled. You can exceed that number if you are foolish enough but if you are in a military organisation and do it for kicks expect a court martial and if you are a civilian pilot expect to lose your license, if you survive of course.