They don't like the idea of a valve between the gray and the black because they are afraid of stuff from the black washing back into the gray tank after the initial flooding of gray water.
As for the elbows, I'm not a believer in them. In the below video, the elbows are attached in optimal conditions. Much closer than they would be to any RV's black tanks. NO RV Black Tanks are going to be as close to the elbow with the hose turned on full force as they are in this video. And they frankly did little to nothing to change what was left in the black tank.
They DO give you a clear view of clean water coming back out making you think your black tank is now beautiful. I think they're a magicians trick.
After dumping, I pull away from the dump station, go in the camper and hold my foot on the flush until I think I have about 2"-3" of water in the tank, throw a pod in there and call it a day. It will continue to help break any remaining solids down. No pyramids or black tank issues or smells in 5 years so it's worked for me.