Service

1) Waist muscles? You don't need to worry about torso rotation on a serve, for the most part. It's all in your wrist/forearm.

2) If you picture the curve of the ball through the air, you'll understand why the first bounce location changes for sidespin vs backspin. A backspin ball slows in the air, so as it leaves the table it can have a low angle, but as it lands, it is often nearly vertical. Because of this angle change, the bounce is usually away from the net. It is possible to hit a "ghost" serve, where the bounce is a little higher and closer to the net, but in this case you're hitting the ball upward and out instead of down into the table.

A sidespin serve bounces with a normal parabola on the y-axis, but it may curve left or right in the air. Thus the ball can bounce a little closer to the net compared to a backspin serve, and then the curve to the side makes the serve look even shorter.