You have made some very good points.
I will talk about the final point you made…
If you look at the LJ today and of 20 years ago i do not think you will find a major difference with speed at take off. There are many jumpers (most of the top 20) today who jump 7.80-8.20 run 10.5 m/s +. This is no different to 20 years ago. Powel and Lewis were outliers of course as they were extremely fast. But at the same time Rutherford of England runs the same speeds from 11-1m as Carl Lewis did and jumps 60cm less on average…How can this therefore be a speed issue?
I feel many more athletes have the potential to increase power far more than speed. Therefore with the LJ once you have got to 10.3 ish m/s (achievable by most talented enough athletes), i feel more elasticity and power work will improve jump distance over more speed work.