Because it may have been a captured Kuiper Belt Object, or possibly an early collision when the solar system was forming. Plus Pluto is only tiny and would be influenced by many gravitational perturbations etc
Why single out Pluto? Eris, which is very nearly the same size as Pluto, has a much greater inclination (44 degrees), and many asteroids in the main belt have inclinations equal to or slightly greater than that of Pluto. It isn't exactly unique in that respect.
Mm. Yes and no. Orbits that are not in the plane of the system are not as stable, but mostly because they have a higher likelihood of colliding with other objects. The creation of a planar solar system is more a process of elimination than a process of conformation. I'm not sure if there is a restorative force in the plane of an orbit.
The other planets will exert a torque on the planet's orbit, but this will result in the precession of the planet's orbit rather than aligning it. Would be similar to the effect the moon has on the Earth. The Moon exerts a torque via the equatorial bulges of the Earth because the Moon does not orbit in the same plane as the equator, and this results in the "wobble" of the Earth's axis.
Two reasons: 1. The orbits do not intercept. While Pluto does spend some of its time closer to the Sun than Neptune and the rest further away, because of Pluto's orbital inclination, when it reaches the point where goes from one to the other it is either above or below Neptune's orbital plane. 2. Neptune and Pluto are in an orbital resonance. This means that their orbits are locked in a pattern relative to each other that prevents Pluto from ever getting very close to Neptune. So even when Pluto "crosses" Neptune's orbit, Neptune will always be at a different point in its orbit.
