if one layer drys, you have to sand. but you can do as many layers, at once, as you want as long as the previous layer isnt dry.
use mat, not cloth.
you dont need fiberglass inside the box most of the time.
its not about coats of resin. you just need to get the fiberglass wet. you dont want gobs of resin. using a cheap paint roller works very well to apply an even coat of resin and push out all the air bubbles.