Borris,
There are many folks who take showers regularly in the CD 25 and TC 255 heads. We find that the TC is easier--mostly because of the head room for those of us who are tall. For me, the easiest way to shower is to sit on the commode (plastic sheeting over the seat may keep it dryer and prevent the holding tank from filling up the holding tank. We also have a shower curtain on the inside of the doors to prevent water from leaking out into the cabin--some boats have gaskets on the door.
As to the fittings: First C Dory has used Bostick sealant on the pipe threads for some reason--this is not a good idea, as you note. Second--this thread is British Pipe Standard Thread, instead of American Pipe Thread standard--they are close, but not identical, and will leak. Ours broke, and the factory sent us a new fitting. We replaced them with no Bostick and they work well.