Thanks for the comments
I believe Mach Crit must be at least equal to, or greater than MMO, otherwise the increase in drag from the shockwave formation, would make operation close to MMO un-economical.
The Mach trimmer is in effect only needed quite a bit after Mach Crit, but since some aircraft have a very slight pitch up just before Mach Crit, and the tuck under, and the fact that the system is available, most aircraft start to use it at a lower speed.
If I remember correctly, the 737 classic is limited to M0.74 if the Mach trimmer fails, so that must still be below Mach Crit.
I would guess Mach Crit to be at about M0.9 on most of the modern airliners.
Just for interest sake, the Spitfire had a Mach Crit of nearly M0.91. It was only because of the extremely effecient elliptical planform. It was not a specific design feature.
If you ask why a person wants to know a bit more about what keeps your aircraft flying, I guess you have not been flying for very long.
The first time you get an emergency that is not covered in any of the books, you will wish you knew a little more. And when you get back on the deck you will start to read up a bit more than the bare minimum required by law.