2" would work, but I would insist on 2.5" for all three. There is a trend on this site recommending no main drain. I like being able to balance the waterflow. My water at 9' deep is cooler than the 92 degrees it is on the surface.
Same problem here. Even with a new gasket on the diverter, it still leaks. I'm about to switch to a cartridge, but for now I put a pipe plug in the back wash line. I take it out when I need to backwash.