The 100m dash have three phases, acceleration, top speed, and speed endurance. Almost all sprinters deaccelerate at the last 20m. Bolt had very good speed endurance, probably because his 200m training.
https://en.wikipedia.org/wiki/Bioenergetic_systems#ATP-CP:_T...
Yes. They train to exert the maximum amount of effort in order to optimise their time. If they're at maximum speed at the finish line it does them no good, they likely could have expended more energy to be quicker in the middle of the run to cut their time.