Is there any event which fires prior to physics forces being applied so I can check their magnitude, and turn off isKinematic if I want my vehicle to get tumbled about?
As far as I know there is no such event that fires prior to a collision. I believe a collision has to actually occur before the data about it can become available. However, there is still a way to take advantage of the collision that occurs to your car while it's kinematic, and base a new physics force off of it.
Just off the top of my head, you might try testing the magnitude of the blast that hit the car's hit box, sending a signal to your car saying it just got hit (if the magnitude came back as "significant") and the magnitude of it, then turning off isKinematic and creating a new omnidirectional explosive force placed at the location of impact with the same magnitude of the one that just hit it.
Should work fine, even if it does sound a little hacky.