Yes, you do need the third valve to operate the upper jaw of the grapple. THe control (on my machine) is located right in front of the control for the 3 point. I've seen integrated joysticks where you can control the 3rd valve from the joystick, which would be nice. But to be honest, I don't think it slows me down that much - the controls are only about a foot or so apart. Of course, unless you have two right arms, you can't operate both controls at once. Like anything on your tractor, once you do it enough times, it just becomes natural and you don't have to think about it.
As for ballast, I have my R1 tires filled with Rimguard, which will give you ballpark 800 pounds (give or take a 100). I usually carry my box blade on the 3 point for a little extra. You could leave your MX6 on the back, but for me the rotary cutter gets in the way when trying to back up in tight areas. With my set up, I can go beyond my loader capacity and it won't lift my back end. Just keep your loads low to the ground and don't drive like Dale Jr.
