I only have the existing system, unchanged now for several years. I can send sketches of that if you like.
But I might build the one you show though I don't have my Hameg where I'm living st present.
My plan now is just to add the zener as you & Alec suggest, to get a faster field decline.
Summarising, I have two probs at present. One, if I reduce the Time for "Fire", the solenoid doesn't Open the "mushroom" and I need to increase Time to about 4 ms minimum. It should act much, much faster. (The mushroom describes a small ferrite piece - the solenoid acts on the flat top, and pulls a "stalk" up within its cylinder housing, exposing a bleed hole to a neck in the stalk).
Two, it is slow to close, so too much fuel goes in. (Two)is what we are dealing with in this dialogue and I think you have solved it for me.