"But for Jules's incident, it's not the same, because as a doctor explained to me, it's not that he took something on the head, the deceleration was too big and a closed cockpit wouldn't have changed anything.
This effectively confirms Jules never took a blow to the head and that it purely was the deacceleration.
I'm all for a closed cockpit since it could have helped avoided accidents like Surtees and Massa, but in Jules' case the big issue was deacceleration. Sadly the car hit the truck on a place where no crash structure was present to soften the blow. Unless you are able to design a closed cockpit which somehow can also act as a crash structure.