ABC and ABN engines work by the same principle. Yes, I am aware that OS using ABN to save a few bucks and make some extra cash. (Nickle being less expensive than Chrome in general)But the expansion principle is exactly the same, the liner expands faster than the piston. Nickle and Chrome are simular metals when it comes to thermal properties, which is why OS can do it this way. OS engines, in particular, are known for having an interference fit when the piston is at TDC, very very tight.

When you talk about the piston being just above BDC, do you mean on the downstroke, or on the upstroke? On the down stroke, just before BDC, the engine is in the intake cycle, and the pressure created in the crankcase is what is pushing fuel up the transfer ports and in to the cylinder. You're saying the back pressure helps push the piston down, but it also keeps the fuel-air mixture from coming up the transfer ports, not a good thing. Too much backpressure, and fuel shoots backwards out of the carb.

On the upstroke, the piston is going to create a vacume in the crankcase that draws the next fuel charge though the carb. I don't think this is what you're talking about.

The earliest throttles were nothing but an exhaust restrictor, no carb at all, because producing back pressure slowed the engine down and acted like a throttle. I don't know for sure why there were engines with both the carb and exhaust cover, but I can think of several reasons why it might be a good idea that aren't related to creating back pressure itself. I'd think it would be because it improved fuel draw. With the introduction of mufflers that provide pressure to the tank, exhaust restrictors went away.

I haven't tested it, but I'd be willing to bet that an engine will idle better with out the muffler than with it, assuming you remove the pressure line to the tank when the muffler is installed. After all, all those big gasser 2-strokes have been running with out a muffler at all for a long time. The're only going to mufflers now because of the noise.

The only time back pressure helps the engine run (other than using it to pressureize the tank) is with a tuned pipe, and that's a different story all together.