I felt obligated to read what you wrote.
Surprisingly,I think most of what you've said here is correct. I'm not sure why you disagree with me on the function of the expansion chamber? All pipes draw vacuum, the expansion chamber just reflects the wave back. Perhaps you think because the charge is richer than it would have been without the expansion chamber (you recover the fresh charge escaping out the exhaust) it burns faster?
If you think about it that's actually correct, but because the pipe makes it richer. You must know taking any pipe off a 2 stroke leans it out.
Dude you seem like an intelligent guy. Honestly I think you're caught up in what makes an engine run lean or rich and not looking at what I said. I'm taking about the charge itself. It's lean or it's rich as a result of all those things, mainly jetting. I swear to you a lean charge actually burns slower.
I think the engineer explained it best, but the burning of fuel in an oxygen rich or oxygen poor environment leads to byproducts that slow or speed the rate of the reaction. You actually get different concentrations of byproducts based on the stoichiometric ratio which dilute the charge, nitrogen being the key one.
I hope this helps clarify things. (edited)