I noticed that the demolition derby scenario requires a minimum of two human players. Is there any way that I can run solo against three other computer-controlled opponents?
You could risk trying to set them to chase mode. That would be pretty difficult to survive. Or You could do this... I have been lately running multiple ai cars and switching between chase, random, and flee mode. I usually start them all at chase, then after they all are piled into each other I will put some to random, disable some, and have the rest flee. After that I put them all back to chase and enjoy the carnage. Its a bit of work but its fun (to me atleast) lol. Obviously with the 2nd way, you can't wreck, its a ton of work to make sure they all wreck...the right way
Are you referring to the multi-seat scenarios? If so then you could use this mod https://www.beamng.com/threads/demolition-derby-the-fairgrounds.35294/ When you start, press shift-c then move the camera to a similar position with wasd and right clicking the mouse to rotate.