I'm saying pick an inertial reference frame in which to look at the problem. You can choose any reference frame you want, but an inertial reference frame simplifies things greatly (the reference frame of the glider is NOT an inertial reference frame because the glider undergoes accelerations). Although arbitrary, the earth-fixed frame can be considered an inertial frame from which to look at the problem (you would be correct to say that an earth-fixed reference frame isn't truly inertial, but I don't think the Coriolis force is significant here). In the earth-fixed reference frame, the "windy glider" gains significant kinetic energy when it turns downwind. Are you saying the "windy glider" doesn't REALLY gain kinetic energy when it turns downwind? If it doesn't really gain kinetic energy then it must gain something else that will cause it to break into little pieces if it hits the ground.

Here's another way of looking at essentially the same problem. Suppose a car accelerates from 0 to 100 mph (on a level surface in the earth fixed reference frame). The kinetic energy that the car gains comes from chemical energy stored in the gas. You could look at this problem from a second reference frame where the car is inially travelling at 100 mph and then accelerates to 200 mph. Viewed from the second reference frame, the car gains more kinetic energy during the acceleration than it did when viewed from the earth-fixed frame (it gains three times as much kinetic energy). In both cases the car should burn the same amount of gas, so where did the extra kinetic energy come from in the second reference frame? The answer is that although the force exerted by the ground on the car is the same in both reference frames, the energy supplied by the the force is different. The energy added to a body by a force acting on it is equal to the force times the amount that the body is displaced by the force. In the second reference frame the force on the car from the ground acts through more displacement (three times the displacement) so it adds more kinetic energy.

In the case of the "no-wind" glider, the unbalanced force that turns the glider through 180 degrees doesn't add ANY energy because the glider is always moving perpendicular to the force. In the case of the "windy glider", there will be a component of the glider's motion that is parallel to the unbalanced turning force (this is easiest to see when the glider has turned through 90 degrees). It is this combination of force and displacement in the direction of the force that adds the energy to the "windy glider".

Consider opposite case where the "windy glider" starts downwind and then turns upwind to a "hover". Where did its kinetic energy go? In this case the unbalanced turning force actually takes energy away because there is a component of the glider's motion that is opposite the force.