Starwoes, simply pulling definitions from an enyclopedia does not really help a lot into understanding the principles involved.
While all you say is true, it does not matter at all in the "real world", as the exhaust velocities on modern jet engines is way way supersonic anyways (as i stated over 800m/s static WITHOUT reheat on the old F-4, thats over Mach 2 at standard atmospheric). No need to change anything when passing supersonic...it will work.
So the airspeed is no factor to be taken into the equation....(at higher airspeeds there are also a lot of changes in mass flow within the engine itself, thus creating a larger exhaust velocity automatically as you go faster....just a remark at the side).
Nozzle control is based solemly on engine internal data, mostly on EGT....
Rocket motors are a totally different chapter, but that is also a propulsion system on which i don't know my way around....

Nick, the "con-di nozzles" also work on the principle of variable area. The reason they are used is not to match ambient and plume pressure, but to increas exhaust velocity tho the most extent possible without pushing the turbine section out of their design envelope (=turbine surge, can be seen when bending the exhaust cone on your turbine real "tight"...homebuilders are always struggling to find a good compromise between high thrust and good running characteristics, eg. good spoolup and low EGT). The convergent-divergent design is neccessary because of the characteristic supersonic aerodynamics....a "simple" book concerning the basics is about 200 pages long, though....


Regards
Hank