They crashed a 100mph car into a wall, a 50mph car into a wall, and then two 50mph cars into each other.

I suggest watching the videos to visually see the proof. The 50mph vs wall is similar to two 50mph cars crashing into each other.

32mph vs 32mph car would have a very low chance of fatality, although the two cars would probably be totaled. When I say ~65mph as a problem, that is the 65mph vs 65mph cars hitting each other head on... or 65mph vs a tree / wall / bridge (which has an equivalent chance of death)

Remember, the kinetic energy of you and your car goes up with the square of your speed.

The reason 2 cars hitting each other is similar is more due to their mutual crumple zones, so they're helping each other slow down at the same time.

I suspect this would also be dependent upon the car, since newer cars have better crumple zones to help survive accidents better. There's a lot of details that go into actual damage. Most people, when they're in an accident, don't go headlong into a wall. They hit a pothole and spin out, or nudge another car and go spinning, or hit a side wall at an angle, where their speed toward the wall isn't actually 65 mph, and so on.

At the end of the day, it's not that 65mph is the near-death number, as far as car speed is concerned, it's more, as everyone else has been saying, the delta-v. The easiest way to hit 65mph in that way is hitting a tree head-on, or wall head-on. On the freeways, this isn't likely the case, and even the edges of roads where two roads split and there's a barricade are protected by several crunch zones in the road itself.

Edit: ah, I didn't notice that you had been saying delta-v as well. Either way, I still think I follow what you're saying now