Not a lav technician, so this is just opinion...
1. Lavs are designed to be robust, no telling what gets dumped down there, so I seriously doubt blue juice will destroy a vac lav. I assume the mx manuals have some rinse out procedure. You would just be making a better smelling (matter of opinion) lav with 2 gallons of the tank filled before departure.
2. I've been flying for 3 decades. I don't micro manage the lavs. Either they work or I call MX out. I ask the lead FA if there is anything they need me to get them before we are ready to push back. I leave it up to them to tell me to call the cleaning crew and have the lavs dumped. The lav status is not displayed up front, just on the FA panel near the 1L door.
Aviate, navigate, communicate.