HarryMann;
Yes, you would be correct in that statement - warmer air, fewer molecules. If the air suddenly warms up, until things stabilize, Mach decreases, (speed of sound decreases, so relative to that, aircraft Mach decreases), engine power is reduced, low-speed buffet onset speed increases.
But I would offer that in and of itself, such a change, (while rare) would not cause loss of control. Engine thrust would increase immediately as per design and as you say, the mass would take some moments to increase velocity. In taking a look on the Jepp CR2, a 20C temperature change makes about a 15 to 20kt change in TAS, (the Jepp assumes "steady state" of course so a sudden "rise" in OAT shows a 20kt increase in TAS but in fact, until things stabilize at the new "equation", there would be an airspeed deficit), and pitch attitude would decrease very slightly first as "CRZ ALT" mode is a "soft" altitude with a 100' window to wander 50' up and 50' down to correct minor speed variations without changing engine thrust.
BOAC;
A 17C increase in OAT under the conditions AF447 was in wouldn't make a dramatic difference in and of itself. The airplane was a long way from the so-called coffin corner everyone is on about. I haven't seen any data to the contrary, proving otherwise. Here's the chart you're referring to, I think. Below this one, here is a graph of the environmental envelope for a 330, (no center tanks):
The following caveat/caution will be familiar to most in aviation. It applies to all readers and is necessary to ensure clear understanding as to intent and use:
The following information is for education, demonstration and discussion only and is not valid for flight planning. These graphs are not associated with any specific airline or model of A330. Do not use these graphs for planning or flight operations purposes.
ISA vs Altitude, M0.80 & M0.82 Cruise:
Here is a buffet boundary graph for an A330:
