Thanks for the answers. For the first part, I don't get the bit about two things with opposing velocities colliding with the same energy as one of them colliding with a stationary wall. That would violate conservation of energy. That is assigning a KE of 0 to the other car. If your point is...
A google search revealed an often asked question (including a locked thread here) about two ships approaching with closing speed > c, say each going .75c. And there's always multiple answers from the experts about relative frames and Lorentz this and that and you have to use the special formula...