Normally the only water that will get behind the fiberglass wall is from that which is splashed on the wall and drips down and wicks through a hole. The spout is usually caulked around the top 3/4 or so to prevent that from happening. There isn't anything behind the fiberglass, the assumption is that it is waterproof. You'd probably find bare studs, and the flanges of the thing screwed or nailed (or maybe if it is panels, not a one-piece) glued to the existing wall (which would require something to support the wall). The push-on tub spouts only use the set screw to keep it from turning, it has nothing to do with the water seal. Tightening it too much could dimple the pipe and make it harder to put a new one on, I supposed (I don't have any practical experience in this!). My unprofessional opinion.