So from what I am understanding is you want the vehicle to stay at a certain speed while driving? Have you tried using cruse control while going down the pass?
When using the tow/haul mode the vehicle electronics thinks you are towing or hauling. So it keeps the gears in a higher RPM as a way to limit speed. When you apply the brakes it thinks you are trying to slow down while towing or hauling so it downshifts to aid in this process. The vehicle is not looking forward enough to understand that you are not towing and trying to maintain a certain speed down a mountain pass.
When I drive down the pass, I use sport mode and manually drop the gears to help maintain the speed I want. I also use the brake pedal as needed. You can use the paddle shifters as to not distract you from the road. I only use tow/haul when towing my ski boat.