(a) Typically, Mach overspeed is not a problem leading to upset.
caveats - mach tuck and stab mistrim can provide some excitement and was a major upset problem duet in the early jet transport days ... leading to several hull losses. Mach tuck increases the tendency for speed exceedance and a mistrimmed stab can make recovery from the dive very difficult .. especially if the stab trim jack stalls and the pilot doesn't realise what's what.
Not so much a problem now with better design/understanding and operational procedures.
(b) U2 .. fly in a very narrow range
those folks had an interest in keeping away from the bad guys' toys. I had a course lecturer many years ago who had been a US U2 driver .. related tales of watching the bad guys start their final zoom climb .. whereupon he would turn this way or that and watch them run out of puff somewhere over there and fall back towards Earth.
Worked real well for all except FGP, I guess.