Isn't Mcrit just the speed at which the first local flows become supersonic? Wing sweep is designed to increase Mcrit, but knowing Mcrit doesn't help with operating the aircraft... Mmo is another story.
Agreed on Mach tuck with shifting centre of pressure (which is why Concorde has fore and aft fuel tanks with transfer pumps, to alter CoG position).
Not too sure about Mcdr and Mcrit being interchangeable either??