If a driver is seriously injured, the first thing you will see is a marshal run to the car and put his fist in the air - that is the signal that means "get a doctor here". The marshals will do their best to do what seems right, but it's mainly a case of wait for the doctors to arrive and do exactly what they say. The best thing a marshal can do if the injuries aren't immediately life threatening (such as a major bleed), is to make the situation safe. This includes putting out fires, turning the cars off and chocking them so they don't roll away. It's then a case of protect the driver, the doctors and other marshals, and help if a doctor tells you to. Doctors take control, and if they need to take a while to cut the car, they take a while. It's better to spend an extra 10 minutes cutting a car that is safe than to drag a driver out. If the air ambulance has arrived on scene, they are there for when the driver has been evacuated, not when they get there. Once that's done, and the driver is gone, the cars will be taken away to the scrutineers to be investigated behind closed doors.
I hope Billy is alright, I've been involved in two major incidents like that, one of which was fatal. I hope the marshals involved are OK, because it's not nice seeing another person in a lot of pain.