1

I want to know how to measure the amount of torso rotation around the vertical axis during forehand strokes in squash, what the best camera position is for measurement, and also how to measure the angle of the racket on the horizontal plane at the moment of hitting the ball.

2

This seems very difficult. If it's around the world vertical axis you might attach a camera to the ceiling, if it's the player vertical axis, aka the longitudinal axis then I don't really see how you can measure this from 2D video, the camera would need to be somewhere along this axis.