mach buffet which I guess some people call high speed stall
No really. What gums is talking about occurs somewhat about the onset of initial buffet. Not a stall, per se, but does involve flow separation and certainly can result in an upset - similar result for the pilot, I guess.
Low speed buffet at high altitude is when you get supersonic flow over the wing
You might like to go back and re-read up on this one ? Lowspeed buffet is normal prestall buffet. Highspeed buffet is the onset of shock wave related flow problems. Mind you, at normal cruise levels, the two might not be separated by a large range on the airspeed side of things.
Things like why the IAS for stall increases with altitude. Never found the answer to that anywhere else.
One should find this in most of the basic books covering aerodynamics