Re: minimum weight
"But delaying the parachute opening allows the seat to slow down a little??? "
Yes; as ejected, the seat is moving forward through the air at the same speed as the aircraft but is nowhere near as aerodynamically shaped. Hence, it slows down as time goes by. Opening a parachute on a forward moving seat will give a large force that causes the pilot's head to move forwards.
However, I still don't understand what the pilot's weight has to do with it. Unless a lower weight pilot results in a faster ejection (upwards) speed due to a lighter mass load on the ejection mechanism ...... I still don't get it.