Correct me if I am wrong but I always thought that coffin corner was where the critical Mach number and stall speed met?
The U2 was also a straight winged jet but operated in that zone to such an extent that the only way to fly was on autopilot?
Why do you say the citation which is a straight winged jet will not experience those problems?
You are correct in your definition of "coffin corner."
But...
Critical Mach number in a modern jet is higher than your Mmo and with a properly designed wing the effects of exceeding it will be diminished compared to older jets...not to mention all the bells and warnings you will hear before you get there. Also, the stall speed of a modern, properly designed wing is much lower even during high altitude cruise so the operating envelope can be very wide even at FL410...thus you should never approach "coffin corner" even at max certified altitude.
For example, the "coffin corner" of a particular modern jet might be a problem at over fifty thousand feet while the aircraft is only certified to operate forty one thousand and you may be hard pressed to climb even that high under normal conditions.
The SR71 operated at around seventy thousand feet and had high performance engines to take it there. Yes, such airplanes are very difficult to fly by hand under those conditions. A Learjet 20 series operating at forty five to fifty thousand feet for example....an old jet with a less than well designed wing (by modern standards) was very difficult to hand fly and you had only a few knots to work with sometimes.
If you are limited in performance by having one of all of your gear extended you would be forced to operate at a lower altitude and lower airspeed. Stall speed may be a concern if you push your altitude too hard at a low airspeed (some guys stalled and crashed a CRJ way back by trying to push it to FL410) but over speeding would be more difficult especially if you adhere to the gear extended speed limit of the AFM.
This is why I say "coffin corner" would not be an issue in the context of this discussion. Hope that helps.
