Don't over-think this. If you are towing any high waled trailer, or any trailer over say 3000 pounds, I recommend trans Tow/Haul mode. If you want to slow down, apply the brakes with enough pressure to get your target speed. As wing_zealot said, the downshift feature is to assist you, not take over decelerating.
I have not heard of any automatic transmissions have caused the engine to go over redline and caused damage in vehicles made in the last 20 years. The computers will simply not allow the downshift to happen under any operator inputs.