Lot of theories discussed, and I'll offer my experiences.
I've had all of the major brands. Moeller, etc. Academy has sold me 2 or 3. The newest stuff (since 2008) is unvented, so pressure is going to build inside the tank, period. The aftermarket tanks are mostly garbage. Yamaha tanks are the only ones worth having, IMO.
Gas. What it does is evaporate. When it does, it's kinda like saltwater. Leaves the junk behind, which is what is left in the tank and which is what the motor tries to burn. Junk. Premium fuel. Burns dirtier than 87, due to the additives that are put in it that are "supposed to" clean the engine. Gas station companies want you to believe that it "cleans" everything but it's just the opposite. Every engine I've had apart that's run premium all it's life, is carbon'd up horribly-rings, intake valves, combustion chambers, etc. If the motor runs fine on 87, and doesn't have a high-test recommendation from the manufacturer, run 87 and call it a day. The new ventless tanks won't let fuel evaporate as quickly. They'll still vent if pressure gets high enough, or that's how they're "supposed" to work. Not all of them do. While on the subject of evaporation of fuel, as it evaporates, it cools. When it cools, it draws moisture in out of the air. Especially in humid climates like we have here in the South and East. Grab you a glass jar, pour it half full of FRESH gas, mark the level with a sharpie, and set it to the side in the shade. Come back in 8 hours and you'll see what I'm talking about.
Ethanol. I ain't seen any yet that's ethanol free. Not that it makes much difference since most engines since the late 1990's will run fine on up to 10%. The government doesn't require the 10% labeling if it's 7% or lower, so some oil companies have taken advantage and added 3-5% and labeling it no ethanol. Only way to know is to test it yourself, using a test strip (available online).
Prevention of carb/injection problems. Run it once in a while, even out of the water, about once a week will suffice, for a few seconds. That's all it needs to circulate a little fuel through the system. Keep fuel fresh if possible. If it's going to sit a while, dump the contents into a lawn mower, or your car, or whatever, as long as it gets used up. Doing those will almost eliminate fuel system related problems, based on my experience.
TCW-3, I think most 2 stroke oil is 3 now, probably has been for a while. Big deal. Actually I remember it going to 3 way back into the 1990's when I sold/serviced personal watercraft (which at that time were all 2 stroke). Glad I got out of that business. TCW3 is just a marine specification, nothing more. I have heard of guys complaining about it too, but the same guys are running month+ old fuel (which might have started off as 50:1 and after evaporation is now 30:1, or whatever), the oil itself evaporates a little too-or some of it's contents do, again leaving junk behind (dirty burn). Then finally some motors are designed to run 100:1 gas/oil, and a lot guys are dead set on running 50:1. You bet it's going to dump some unburnt oil oil, it's running double the amount of oil it was designed to. Gum up rings, black junk runs down the foot out of the prop, etc. At one time there was talk of TCW4, but I have not heard of any further announcements, the last time I asked, it was made known that 2 stroke outboards are a thing of the past and thus the 4 classification was not really needed, as manufacturers of the high tech 2 strokes specified their own lube oil, specifically designed for their engines so that it meets EPA standards.