ORIGINAL: flythesky
Let's consider this as an object moving thru space at an absolute speed and direction, lets not talk about airspeed yet. In your diagram the plane is moving up at a speed of 20 kts and has a certain amount of energy from it's motion, it is then directed to the left, loses 5kts and a portion of it's energy in the turn, and then proceeds to the left at 15kts with the remaining energy. To say otherwise is to say that it somehow instantly lost the energy that it has. Now we consider air speed. Moving to the left at 15kts it encounters air moving to the right at 20kts. The result is that air is moving over the plane at 35kts. If we can agree on this we can continue to analyze the rest of the scenario.
The bolded part seems to be at the root of your difficulty. As mentioned by someone else already the model doesn't "encounter" wind and add it to the airspeed as you suggest. In a steady wind the model flies at a steady speed when flying in a straight line with its 15 knots of wind passing over the wings. And in actual fact a 15 kt flying speed into a 20 kt wind would result in the model drifting backwards at -5kts ground speed.