engine brake while towing?

This is false. The pressure control solenoid/valve can increase and decrease the amount of pressure coming from the pump to the transmission. Again, think of a garden hose and putting your thumb over half the end. BOOM, you just became a pressure control valve increasing the pressure output of the hose even though the pump did not increase output. So yes, the pressure control solenoid/valve can make more pressure coming out of it than what the pump is putting in.

The pump and control valve are separate components with different functions. The valve can only work with what the pump gives it. When you put your finger over the end of a garden hose, you are increasing pressure at the expense of flow. As soon as you remove your finger, flow increases but pressure decreases. A pump cannot increase pressure and maintain the same flow unless it increases speed to pump more.