As a light tank the Tetrarch was quite good. At the time of it's intended production (delayed due to France debacle), it's competitors were the Pz II, L6, T-40 and M2A4. The Pz II and L6 only had a 20mm auto-cannon and the T40 only a 12.7mm MG.
As for turret layout, having commander double as loader is a less concentration intensive combination than commander doubling as gunner. The commander-loader can still stick his head out between loading without the gunner having to take his eye away from the gun sight.
Pz II: The 3rd crewman was the radio operator sitting to the left rear of the hull, not in the turret. He could hand up a 20mm magazine to the commander/gunner but the commander was commander-gunner-loader in practice. ASL should have made the Pz II a 1MT.
Pz 35(t) and 38(t), though mediums, as the LT-35 & LT-38 started off as 1MT and the Germans added a loader, the commander still was a commander-gunner so should both be RST.
L6: 1MT, 'nuff said.
The T-40, T-60 and T70 were 1MT, though the T70 at least had a proper gun. The '43 T-80 finally achieved ST with a gunner and commander-loader.
M2A4: The gunner's position was to the left, but so was the commander's angular cupola, implying a commander-gunner and a loader. Dropping the cupola from the M3A1 meant that the commander could move to the right (same as US mediums) without disadvantage to make a commander-loader and gunner division of labour. So the M2A4 and M3 should be RST while M3A1 and M5A1 would be ST. The M2A4 and M3 are ones that are a little less clear, but only having a cupola hatch meant that even if the commander doubled as loader he was greatly vision restricted. The M3A1 and M5 had two turret hatches, so not the same problem.
Had the Tetrarch started production in July '40 as intended it would have been only inferior to the M2A4 or Pz 35/38(t) (mediums) in armour thickness. It's turret crew arrangement was superior to all the above at that time. It had a decent AT weapon. By the time of it's production in late '41 it was indeed obsolete, like most light tanks.
As for it's airborne role, clearly many armies wanted such vehicles. The US had the M551 that eventually proved too heavy but the Soviets finally achieved decent success with the BMD series. While airborne forces achieve operational or high level tactical mobility by aircraft or helicopters, low level tactical mobility gives the airborne force a chance to exploit fleeting opportunities offered by a temporarily surprised or disorganised enemy. At Arnhem, the jeep mounted forces managed to get to the bridge, few of the walking did.