Can I use _x and _y values that are less than 0 or greater than the width or height of the work area?
Yes. You can use almost any reasonable value for _x and _y. You can position movie clips off the edge of the work area. You can still see them if you adjust the test window properly, but they will be partially or completely hidden when played back in the browser.
Is there any way to detect the mouse location when it is outside the work area?
No. But you can use the technique of making the work area much bigger than what you need so that there is some extra space around the edge of your graphics where the mouse location can be detected.
When you rotate a movie clip, what determines the point at which it rotates around?
When you edit a movie clip in the work area, you will notice a small cross at the center of the screen. This is the movie clip's registration point. This point corresponds to the location of the _x and _y properties of the movie clip and the point of rotation.
If a movie clip already contains a semitransparent graphic, does the _alpha property override it?
No. The semitransparency of the graphics and the _alpha property are combined to determine the final transparency of the movie clip parts.