Well doesn't that mean that all planets are in the same plane except Mecury, Earth, and Pluto? The same except those extremes.
Pick a coordinate system and stick to it. Since we are on earth most of the time, we pick earth's orbit as the zero-zero. If we were on Mars we would pick Mars' orbit as zero-zero. Or, we could pick the plane of the sun's equator, which until recently was not known as precisely as earth's orbit. Pick one.
Not quite. The solar system formed out of an accretion disc causing most planetary formation to be close in orbital inclination. However, there are minor differences between all of the planets; Pluto and Mercury being the most extreme.
Note, this does not directly equate to a planets eccentricity or axial tilt referenced to the Earth's equatorial plane.