We owned an HVAC business for a number of years, sold it to the guys that were working for us when we moved out of the area.
First off, it's not possible to tell anyone that you need this much cooling for this much area unless you have done considerable research on heat load, infiltration of air, space usage, insulation and a slew of other boring stuff. A tech that has been in an area for an extended period of time may be able to make a close guess but that would be all it would be.
In your case it sounds like you have a very simple solution to the problem, couple of them really. First you could put a lock on the T-stat and break the fingers of anyone that messed with it. This usually leads to divorce and lots of lawyer bills.
Easiest solution is to buy a programable thermostat that allows you to put in "set back" period during the 24 hour day. Set the temperature where you want it to be for the day and night periods then go back in and set a 10 to 30 minute period where the temperature called for is 80 degrees. Set one of these high temperature periods every four, six or eight hours as needed to allow the AC unit to defrost the coil. Set the fan to run full time then when the compressor shuts off the warm air from the house will be pulled over the coil and any frost/ice accumulation will melt off. Be sure the condensate pan under the coil is draining properly as having the coil setting in a puddle of water is a sure way to ice things up.
The length of the set back and how often they must occur you can experiment with and find what works best for you, the humidity in your area and the capacity of the machine.
I find it odd that the tech said this was caused by undersize ducting as that is seldom the case. Usually the ducting is sized to the fan on the system. If the fan is multiple speed it may have been set to a speed higher than originally specked or the entire system may have been upgraded and put on the old ductwork.
If you are consistently finding the house warmer than you want it then your ideas of venting heat from the attic, fan forced systems are better than turbines, and addition of spot cooling with a window shaker or two are viable options and way cheaper than changing the central unit.
Also consider adding outside bamboo blinds or such to stop the heat of the sun before it gets to your windows. This is one of the easiest, and least expensive, methods of lowering heat gain in a building.
You can try to explain to "someone else" that setting the stat really low will not make the unit cool any faster. You can try but it probably won't work.
Hope this helps, any questions I'll try to answer.
Mike