Press the 'V' key to cycle through the four different colored vehicles.
No, the customization options are limited to color changes for the current vehicle you are using.
Use the arrow keys to steer and spacebar to jump. Press 'W' for a boost during the jump to perform tricks.
The objectives vary but generally involve reaching checkpoints, completing missions, and achieving a high score by performing stunts and maneuvers.
Unfortunately, Formula Crazy Stunts does not support multiplayer mode. It's designed as a single-player experience.