I think it may be all to do with gravity. I think that the size of an object affects how much gravitational pull it has, hence why we rotate around the sun, because its bigger than the Earth. I think it may be impossible for the moon to touch the Earth also, because it has found a comfortable distance from us, without having entered our atmosphere, where the gravitational pull would make it launch towards us. I think the rotating on the axis and plate tectonics are all by products of gravity. I don't believe the moon had anything to do with the shape of the Earth. Other planets have multiple moons, and are still just as round.
Well, it started off flat, but after hundreds of years of spinning over 1000 mph, the force caused the center of the earth to bulge out... Couple that with the force of the moons gravity causing the points to curve back inward on each other, and the centrifugal force of spinning around the sun, and it was only a matter of time before the Earth smoothed out into a nice uniform sphere.