Depends on the level of the athlete as to how long acc. lasts. Hip extension rules, no matter what kind of start. Yes you are wrong in saying that knee extension rules in acc., its more involved, but hip extension via the hamstrings is more active during all phases.
Check this out http://www.elitetrack.com/wiemann.pdf
Yes, you cannot overload and stress the organism to nearly the same extent with a DB squat as compared to a BB squat. So no their not the same and although the movement appears to work the same, the BB squat would be supierior because it stress the organism to a much greater xtent.
Yes, exactly RnR, because more weight can be handled in a BB squat. There is also a greater stress because the BB is moved through a greater range of motion and also you couldnt hold as much weight as you could load on a BB.
For the first few steps out of the blocks the quads may be the primary movers. As knee flexion decreases they play less and less of a role in producing horizontal velocity. The quads would be the primary mover when the base of support is behind the COG and there is flexion around the knee.