And another explanation...
My understanding of the original post is:
Mcrit is the %mach for a given wing (determined in 1g level flight, at a given weight) at which shock waves will begin to form on it's upper surface.
Mach Number Buffet is the result... and yes, you will feel it, but it could feel like a pre-stall buffet.
Mach Tuck is a negative stability inherant in the design of some acft that leads to a nose down pitching moment at high mach numbers during the approach to Mcrit (and certainly past it). The way i understand this is that the local flow is NOT yet sonic below Mcrit, but TRANSsonic, which does change the airflow and CofP caracteristics somewhat due to drag increases on the wing's upper surface, and therefore gives us the beginning of the nose down instability. This last bit is not in D.P Davies book but is only my understanding of it which may well be wrong. Pls comment...
Mach Trimmers are installed so that the acft can be certified stable in all flight regimes (and outside of them). This is not "true" stability, but an augmented stability which is why it may not be req on FBW acft (as someone said earlier).
For eg, the 737 mach trimmer is active above M.61. If it fails in the classic then there is a limit of M.74 but the NG is only limited to its Mmo of .82. So... the classic becomes negatively stable above M.74, but the NG (like some acft described earlier) remains stable up to its Mmo, but the mach trimmer is still required for certification (due to needing to be certified over and above normal flight parameters eg Mmo).
I can't imagine (assuming i'm right in the above) any std airliner cruising above Mcrit due to the high drag and loss of efficiency that would result, vis-a-vis the shelving of Boeing's sonic cruiser.
HJ