They all affect each other. Another force that might be acting is magnetism. The magnetic field of the sun may have rotated or weakened so it doesn't interact with earth's mag field so strongly.
No. The other planets, and this one,too, were effected one time back about when God said... That effect has affected Earth ever since.
Satellites move in inertial space, about an Earth that rotates. The motion of the Earth since a fixed epoch (currently noon, January 1, 2000) has to be taken into account for things that depend on the satellite's location over the ground -- some gravitational effects, and the location of radar tracking stations.
We have to account for leap seconds, as that's obviously a component (albeit small) in the equation.