Willie, I'm afraid you're making a way more of this than you need to. If you take off downwind you'll take off just fine when it reaches 15 mph of airspeed.

Now if you want to talk about a REAL LIFE downwind takeoff factors that's different. Your model will still lift off just fine at 15 mph. But as you climb up through the first couple of feet you're going through a wind shear zone and the wind at 2 feet is stronger than in the first foot close to the ground. If you try to fly at the same 25 mph ground speed then this shear can bite your model. But if you lift off and accelerate then it's not going to be an issue. This is the same effect you get during landings on windy days. You need to keep the nose down and flying speed up so that as the model drops into the lower velocity air close to the ground it doesn't suddenly fall below the stall speed.

Your wing angles being positive or negative depending on direction don't enter into it at all. That one's a non issue other than how the wind's localized turbulent flow patterns react with the airplane. But that is totally unpredictable and the only defense when it's present is to ensure that you don't lift off until you have more than your minimum controllable flying speed. I've seen windy days when it was like a washing machine up there and other equally windy days when it was very consitent and a non issue to deal with.

But none of that was noted in your original problem. The answer still is 25 mph ground speed if it's a tailwind.