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2 man turrets could come two ways. Gunner and commander/loader and commander/gunner and loader. The latter meant you needed fewer "skilled" tank men but it also meant poorer tactical co-operation between tanks or the units. Again the benefits of operating "unbuttoned" are rather diminished by the commander having to spend so much time with his head inside the turret in order to do his "second" job. T-34s could operate unbuttoned with that big hatch folded up but with the tank commander operating as the gunner it didn't really add anything to the ability of the tank.
3 man turrets are were the ability to operate unbuttoned really show up.
The Czech tank is about the only light tank to have that ability. A few British light tanks do but that is because they are armed with Machine guns and the commander is not needed every few seconds while firing to reload the gun. Of course the 12.7 Vickers can't really hurt anything heavier than a MK II either.
I'd like to return on this part.
Those '99%' of the time the tank was operating (un-buttoned), would exclude fighting vs. ground targets? The tank still can be attacked by enemy air force, being miles from the front line. A tank crew can have its DP gun elevated somewhat, ready to pounce an enemy plane - something like US tanks had (HMG), but better. Commander will be buttoned up when it's time to engage ground targets, or when the enemy ground fire is judged as incoming.
The magazine-fed 20-25mm (for under-10 tonner; 30mm for German under-15 tonner?) might be the best solution for a two men turret of a light tank. Aimer can have between 10-60 shots to dispose with the enemy, prior re-loading, plus the MG, if the target is suitable. If we're going for a two men crew and a single shot weapon, maybe it's the best if both crew members can load the gun. Not as good as 3-men turret, that's for sure.
Perhaps we need to revisit why one would want light tanks if you can have heavier more combat capable ones. What would they need to fight? If a light tank is part of an army with period MBTs then it only needs to fight infantry and individual anti tank guns in order to progress it's reconnaissance duties. If it meets stronger opposition then it's duty is to run away and report that.
WW2 British higher formation reconnaissance policy was to mix low visibility small scout vehicles with supporting support fire from larger ones and to have a small organic infantry element. A lightweight mobile all arms arrangement.
Light tanks were the reconnaissance arms of armoured regiments; having superior mobility to the battle tanks to find the main strength of the enemy. But then to leave engaging that main enemy to the larger battle tanks once the enemy had been found and their positions understood.
I fear that we are trying to make a more combat capable light tank where a larger one would be more suitable, not a light tank that can do a light tank's duties better.
The catch was that many countries were ill capable to build tanks above certain tonnage, and even the most industrialized nations relied to 6-15 ton tanks to double as MBTs. Poland, Japan, Italy - they all made their mark in ww2, yet anything above 15 tons was almost impossible for them to churn out in significant numbers. It was judged that even a 'bad' tank is better than no tanks at all, all belligerents were thinking in those lines. Eg. the German offensive vs. West in 1940 was won, among other stuff, by light tanks.
Light tank (disregarding the operator's nomenclature) was in great demand, for fighting, and prior 1943 it was comprising the bulk of the tanks in use, worldwide. So a better combat capable light tank would come in handy. Once it's past it's prime, it can serve as recon, os a pltform for other duties.