1. Most membranes used on the surface of the board meeting ANSI 118.10 pass very little water. A perm rating of 1. Can pass 38.4 oz of water vapor per 100 sq. ft. Per 7 days of continuous use. ( nobleseal is 0.15 )We are talking showers not steam here.
2. If water got into the wall it would /could , just drain out the bottom of the " vapor" barrier into the pan.