Planet-sized bodies are not billiard balls, tidal forces tend to break up the smaller one even before actual contact is made. I've always heard that if our moon wandered too close to earth it would be ripped apart and the debris would form a ring around us like Saturn. Indeed, Saturn's rings lie in the area where these tidal forces prevent another satellite from coalescing gravitationally from the debris (the so called Roche Limit).

Now maybe Charon is small enough and "sticky" enough that the tidal forces cannot overcome the tendency of its rocks to bind together, both gravitationally and mechanically. I really don't know the answer to that.

But if that were the case, the two worlds would have highly elliptical orbits. Nice, round orbits strongly suggest the system is stable, and either formed from normal accretion during Pluto's formation, or is the remains of an ancient collision between comparably massive bodies. Post-formation collisions and even near misses would leave evidence in the celestial mechanics of the system.

According to the Wikipedia entry on Charon, the eccentricity of its orbit is 0.00, almost perfectly circular to two sig figs! This strongly suggests a well-behaved origin to the system, from hierarchical condensation from some primordial nebula; or a collision scenario similar to the one proposed for our own moon, but no post-formation collisions or major disruptions from external third bodies. A near-circular orbit resulting from a collision is possible because only the fragments with low eccentricity orbits are available for accretion. The others either are ejected from the system or collide with one of the two principals

The matching red stains may be connected somehow, but I don't thing a fender bender did it. The Pluto-Charon system is tidally locked, just like the earth-moon system. It would be very interesting if the two red spots faced each other continuously as the two worlds
circled each other.