But by opening the vents all you are ensuring is that the system will fail to cope.
No, what he is saying is that the system
already fails to cope, and therefore it is necessary to open the vents.
If the system then responds by reversing its behaviour entirely and heating instead of cooling, then it is badly designed. As you have in fact described:
Well, the coach thermostats reside inside the ceiling panels along with the rest of the A/C gubbins... The air blast that enters the coach does so at ceiling height, travelling along the ceiling panels and hitting the coach thermostat. This tricks the thermostat into thinking that the coach is too cold and the A/C switches from cooling to heating.
There are two points here. Point one is that the thermostat is wrongly positioned. It should be located so that it detects a temperature which is representative of the carriage as a whole. Since it evidently does not do this, the design is bad. The thermostat does
not have to be adjacent to the rest of the gubbins; it can be anywhere you can run a wire to, but someone would have had to think about that and they couldn't be bothered.
Point two is that the hopper vents are also badly designed, because they do not direct the airflow to where it is needed, but send what flow they do manage to admit in a direction that does no good (aircon or not). As of course we all know if we have ever tried to achieve decent ventilation using them...
The system is
not "working perfectly"; it is failing at multiple points.
Johnny Q Public comes along and opens a vent for no good reason other than it's a nice sunny day outside.
There are other reasons for opening vents than simply trying to adjust the temperature. It's nice to have a bit of fresh air when the weather allows instead of stuffy stagnant muck. Also, there is a roughly even chance that someone in the carriage will be drenched in some chemical warfare agent; it only takes one, and the choice is to open a window or feel ill all the rest of the day. The loss of decent openable windows is a significant factor in why train travel these days is much less pleasant than it used to be.
Unfortunately with some types of air conditioning systems, they ice up and then fail to work properly if they run continuously.
Yes. Crap ones. It happens because the evaporator matrix is too small and/or the airflow through it too little to achieve the necessary rate of heat transfer without an excessive temperature differential between the air and the metal. Another consequence is reduced efficiency and higher energy consumption in normal operation.
Playing games with the thermostat is
not the right way to deal with this; it just makes an already ineffective system even less effective. A decentish bodge solution is to fit an air flow meter which detects the reduction in airflow from incipient icing before it develops into a total blockage, and shuts the compressor down, leaving the fan running, until the airflow returns to normal, but the correct solution is to have installed a properly designed system with correctly sized components in the first place.
Add in the greenhouse effect of the sun through the windows...
Oh yes, nothing like a bit of directional radiant spot heat to mess up an averaging system. Perishing cold icy winter's day, train heating quite reasonably running full pelt... but it's a
sunny winter's day, with that blasting low winter sun shining full in through the windows and directly onto me, giving me a few hundred watts of purely personal thermal input, and I melt.