Two thoughts off-hand - I may have missed where you included in but if you didn't already, you may want to include some basic filtration between the grey water tank and the pump to make cleaning out the hair and soap scum easier than having to pull the pump screen.
Second, if I were doing it, I'd try to find some way to disable the grey water pump when the three way valve is set to fresh water. Either a physical interlock of some sort or an electrical lockout where that pump could not be powered up until the valve was set to grey water.
Other than those thoughts, it sounds to me like it should do what you are trying to accomplish. As a pre-construction test, I'd go draw a bucket of couple day old bath water from the grey water tank, bring it inside for a few hours, and let DW give it a good sniff to see if the odor is going to be an issue.