A collision of two bodies of roughly comparable size would be totally inelastic, that is, you would wind up with two floating piles of rubble, or a rubble pile surrounded by a ring of rubble, which might eventually coalesce into another spheroidal body. It is thought our own moon formed this way. Although it is certainly possible that the Pluto system may be the result of some great collision, it is unlikely the red smears are direct evidence of that event. They are either coincidentally similar, or they are evidence of some highly complex sequence of events, or perhaps even the intervention of some third body.

Do we know the orbital elements of the various satellites of Pluto? I know there are several others besides Charon. If they are all in neat, regular, near-circular orbits rotating in the same direction, this suggests an orderly accretion-driven formation to the system. OTOH, if they are in highly elliptical orbits, if any are retrograde or in different planes, perhaps locked into complex resonances, this suggests the system may have been formed under more catastrophic circumstances.