Re: minimum weight
They probably used a very, very simple fault impervious and time proven system like a burning fuse lit by the initial ejection charge
They don't. They use a barostat. This contains a clockwork timer and a diaphragm pressure switch. Deployment is delayed until the timer has expired - so that the seat is clear of the airframe - and until the pressure is high enough. If the pressure is too low, then the seat might be too high, or else it is travelling too fast. In either case, it sits under the drogue until it reaches the correct height & speed, and then a clamp is released which allows the drogue chute cable to pull the main parachute out.
It's quite ingenious really.