Kerdi is waterproof as long as you get at least 2" overlap and properly embed it in the thinset. KerdiFix is a urethane sealant. The combination of the two should give you a good seal, just apply a patch (use KerdiBand for less buildup if you prefer). Also, keep in mind that most trim rings also have a gasket to keep water out. Only if you're dealing with a steam shower is it imperative to seal around the valve really tight.