1. It can help you visualize the tile spacing, grout color, and placement to help you decide before you commit to the whole job, but no, it isn't essential.
2. That may be more than needed...I'd have to look it up to be sure, but I would think a minimum of 1/4" per foot.
3. If it doesn't leak in 24-hours, it's unlikely to leak in 72, but if you have the time, it won't hurt. The longer it sits, the more you have to account for evaporation, otherwise, you'll get a false indication when it does drop some. But, it would disclose wicking or leaks into the wall or floor, if you have access to see it (which may not be the case for all sides).