The plumber came today. He told me I didn't need an outflow valve. I pointed out that the manual says it should have an outflow valve.
We checked the shower, the faucet was installed upside down so the water was mixing properly and that was why the shower wasn't working. It didn't have anything to do with the water heater at all.
But I asked him to please install the valve anyway. After the cruise he's agreed to come do it. He agreed grudgingly with a sigh, but he agreed. I'm also going to have him make the outside faucets and toilet tank run with cold water instead of hot. But as long as I know it will happen I've lost some of the flat-out pissed off that I was feeling.