Originally Posted by niallo
Therefore the selected cruise altitude should ensure a safe flight unless a manoeuvre (advertent or inadvertent), or turbulence, results in the aircraft exceeding the margin.
Not quite. The certification regulations require it to be shown that an inadvertent manoeuvre or turbulence should also not result in any "margin" being exceeded. There is a question what degree of turbulence may be found at cruise flight levels, because by no means everything is known about the weather at these altitudes. But the certification regulations try to ensure that, if a disturbance ensues at cruise altitude and airspeed, the airplane will recover itself. As john_tullamarine pointed out, this wasn't always so, that the regulations have evolved as a result of experience, but I think it is safe to say that it is the case now, give or take a little bit.
Originally Posted by niallo
If that should occur, the flight crew is confident, thanks to their training, firstly that they can identify what has caused the situation (eg over or under speed), and secondly that they can correct the situation before severe buffet occurs and certainly before an excursion outside the certified flight envelope, possibly leading to an upset.
....the flight crew is confident, thanks to the airworthiness certification of the airplane, that the airplane can recover itself (= "correct the situation") by virtue of its stability characteristics, before an excursion outside the flight envelope (e.g., upset) occurs.
This is not to say that the certification process is perfect, by any means. But I have described the overall intention and I think that is what has been achieved, give or take a bit, over fifty years of experience certifying passenger jets for flight at these altitudes.
PBL