I'm not sure what setup you've been looking at, but none that I know of have two rocker switches for control of the TnT cylinders, there is usually a spool valve with levers to do that. A third spool can be added and extended to the front with 1/4" hoses for another function on the loader.
For a 3rd function on the loader, often a solenoid valve is used, that is controlled by a single rocker switch (usually on the joystick) and is indeed non-proportional or ON/OFF as you stated, they are plumbed inline with the PB line just as the rear spool valve would be. Truly proportional electric solenoid valves are very expensive and generally only found on commercial machines using pilot operated controls, it would cost thousands to add to a compact tractor.
Another frequently added device for a 3rd loader function is called a "diverter" valve. These divert the dump/curl circuit to the grapple or 4n1 momentarily by operating a momentary switch on the joystick, and you maintain your proportional control with the left/right movement of the joystick. Brian also sells these divertter's, as do I.