Clearing the orbit is done in more ways than by collisions. A near approach allows gravitational interactions that could throw an asteroid out of the orbit or even out of the solar system entirely. In just the right conditions, there’s the possibility that you capture the object as a moon.
In any event, even if we assumed that the lifetime impacts on a planet like Earth is higher than lifetime impacts on Pluto, it would still be the case that Pluto is currently at a higher risk of impact, and that it has been at higher risk for at least three or even four billion years. Earth has already cleared out its zone - most of the damage is done. Pluto is still flying through an uncleared debris field.
The barycenter is closer to the bigger object (obviously). The L1 point is closer to the smaller object (because it doesn’t “tug” as much), and incredibly unstable. Think balancing a pencil on its point.
This is the second time this question has come up here in recent weeks, btw.
It is certainly possible that Charon and Pluto could have a common origin, but it is far more likely that they were either ejected together out of the influence of a larger body (like Uranus or Neptune) or came together in a chaotic capture with a number of other objects which were later ejected taking away excess angular momentum and giving the Pluto-Charon system greater stability. This is unlikely to be an uncommon occurance, and as we explore the Kuiper Belt and observe more extrasolar planets such configurations may be seen with much greater frequency. As Chronos notes, KBOs probably have all kinds of ridiculous orbits and configurations owing to their low mass and large dispersions. They’re more like molecules of water turbulently swirling a drain than marbles rolling around in a dish.
While there was once believed to be either a large hidden planet orbiting somewhere beyond Pluto or a distant ‘dark companion’ (e.g. a brown dwaft) orbiting the Sun which was used as the hypothetical basis for a number of different, then unexplained phenomena in the solar system, most recent models fail to show any conclusive need for or influence by another planetary or larger mass. However, our ability to visually observe dark (non-radiating) objects in trans-Neptunian space is almost nil, and the capability to distinguish gravitational effects from even a brown dwarf-sized object in a very long period orbit with an aphelion of thousands of AU or further from the galactic tides and net effects of the aggregate gravity of the Local Fluff relies on stochastic filtering and critical selection of parameters in a very non-linear dynamic model of our system’s celestial mechanics. in other words, if we go far enough out, it would certainly be possible for a large planetary or brown dwarf mass to exist, but it isn’t something we need to explain current observations of planetary motion.