I cannot answer your first question.
I too use my stove inside the test vestibule and this was one reason for wanting a stable stove. My first effort is here.
To use a canister stove which has a screw thread, inverted or upright, with the camping gaz CV canisters, you need a Markill Valve Cartridge Adapter MK92497-100
Your last question is a good one: there are many remote canister stoves that have a preheat tube which is claimed to improve cold weather performance, but the manufacturers make no mention of inverting the canister in order to obtain this improved performance. Some, like Primus, have the adjustment knob on top of the valve making it almost impossible to invert the canister, so one may assume that it is not designed to be used upright, and yet there is absolutely no point having a preheat tube when the canister is used upright.
My conclusion is that if the stove has a preheat tube, it is possibly designed to be used inverted, but the manufacturer does not promote this for fear of liability in case of any flare-up.
Added: Coleman promote the use of an inverted canister with their Fyrestorm stove, look at all the warnings in the instruction manual!