1.) There are several theories, but nobody knows "exactly" what happens to your muscles when they grow. Tension, and time under tension seem to be important tho.
2.) Again, I don't think there is one, simple, clear cut answer. If you practice the move you get better at it, recruiting fibers better. If you keep practicing the move over and over you recruit more muscle fibers as you fatigue.
3.) Most good programs use different load and rep ranges. You could work up to some heavy singles, then do a couple back off sets of 6-8. Or you could do a big exercise heavy (bench press) a "secondary exercise" for medium reps (incline dumbbell press) and then "assistance exercises" (flies, dips, tricep pushdowns) for high reps to hit all the different fibers.
Over the years lifters have developed all kinds of different plans based on the principles you are considering. High reps, low reps, moving the bar fast, moving the bat slow, assistance, no assistance, on and on and on. Some guys go more by "feel" and some like to go by the numbers/science more.
Regarding faster/overspeed negatives; Louie Simmons of Westside Barbell agrees with you. A faster "down" means more force, resulting in a more forceful or faster up, similar to plyometrics. That is one of the reason they attach chains and especially bands to the bar.
Check out his articles at westside-barbell. There are over 120 articles and many of them talk about using these ideas to develop the sort of "optimal training" you are thinking about. In fact, one is called "Optimal Training."