I didn't say it was a predictor of technological advancement, did I? I said it's easier for a larger population to make headway in technology than it is for a smaller one.
Larger populations allow for more division of labor, which results in greater expertise across all fields of industry. That expertise then leads to technological advancement, which in turn raises quality of life, which allows for even greater population size, and so on.
Similarly, a low level of technology makes it harder to sustain population size, which means that all the people will have their hands full just keeping things running, with no time for innovation/research. Thus the next generation will have just as much trouble as the last, creating a vicious circle.
So, questions 2 and 3 still. Do you agree?
Your hangup is that you believe slaves replace unskilled labor instead of simple expanding the size of the unskilled labor pool.