Damn, i had a lot of typos on that long post of mine, i didn't have time to proof-read it before posting
Azimech and Skoshi are also correct in their information.
As for your last question i think most of the aircraft you are testing have automatic mixture controls in IL2, that's why you can't mess with it even though you have CEM turned on and the keys mapped. The P47 in reality has the semi auto system (cut-off/full rich/auto-lean/auto-rich) but in IL2 i think it's fully auto. As for the corsair, maybe it only has auto and emergency full rich settings (the 120% setting) in IL2.
If you want to run a quick mixture test fire up the quick mission builder, select a Yak-1 with no enemies, set the starting altitude to 5000m and when you spawn set the supercharger to high gear (does the yak-1 even have one? not sure) apply full throttle and get climbing.
You'll see almost immediately that there appears to be something wrong with the engine, it sounds strange and seems to not deliver enough power. Switch to an external camera and you'll see a trail of black smoke coming out of your exhaust stacks. Now switch back to cockpit view, lower the mixture to 80% or 60% (it's done in 20% intervals per key press in IL2) and you'll hear and see (instruments) the engine come back to its senses. Switching to the external camera you can also see the black smoke trails have disappeared.
That smoke is due to the excessive fuel being burned in an inefficient manner (or being almost unburned even). There has also been talk that due to the analog nature of auto-mixture systems, it took a couple of seconds for it to stabilize if throttle was applied abruptly. This had the effect that sometimes if the pilot quickly slammed the throttles forward, a brief trail of black smoke was produced until the auto-mixture system stabilized. In turn, this was used by pilots to judge if the enemy aircraft was suddenly going to high throttle settings, a clear sign that he is aware of you and is initiating combat.