All three of these do similar damage, if the cars have equal mass:
* 70mph car vs. 70mph car
* 140mph car vs. parked car
* 70mph car vs. wall.
The only real question is whether you consider "140mph impact" to mean "vs. a similar-mass car" or "vs. a wall". These are wildly different impact intensities that should not be confused.
Yes, if the cars are identical and collide perfectly enough that both crumple in exact symetry. Each absorbs and so the damage is split, as opposed to hiting a non-absorbing wall. But that is basically impossible to setup.
No. If the wall is stationary and infinitely massive, you’d bounce off at the same velocity; if it was moving at the same speed as you you’d bounce off at three times the original velocity.
Edit: if the other object is a car, then yes, there is no difference in either case from your perspective.
If the collision is completely inelastic, then there will be no bouncing. In a car, usually the crumple zone completely absorbs and dissipates all the energy kinetic.
Mythbusters experiment: https://www.dailymotion.com/video/x2n9j62