Density is a variable in the equation for lift, drag and reynolds number. But the value of density at any point in space and time is fixed for whatever object is in it. Whether you calculate the lift from a insect or from a jumbo you use the same value for density. The density value is not modified by the object you are doing the calculation for. The density is only changed by pressure and temperature, not by the object within the air. The fact that you get different values out of the calculations is due to different properties of the object, not the density of the air which remains a constant value. Therefore there is no such thing as relative density. Viscosity also remains a constant value but viscosity is a resistance to shearing by the object. Air viscosity and air momentum can oppose one another, when air is accelerated and curved around the upper surface of the wing its momentum wants to make it go in a straight line and detach from the curved surface whereas viscosity wants to keep the air attached to the surface. What happens depends on which has the greater force at any point. For example the stall occurs when momentum carrying the air away from the wing overcomes viscous forces trying to keep the air stuck to the wing, and the flow detaches from the surface. But small objects impart very little momentum to the air so the air's viscosity effects dominate over the opposing momentum. Thus the air has the same viscosity for any object but the counterbalancing momentum force that is created by the object can be very different depending upon the size of the object, so small objects feel all the viscosity and hardly any counterbalance which is why small objects feel as if viscosity is much greater.

While thrust of some form is required to give the glider height in the first place, it plays no part in the generation of lift. Lift is 1/2*density*V^2*Cl*A. There is no variable in that equation for thrust, it is not thrust that generates the lift. Thrust is needed to overcome drag in order to maintain V above zero but it is not the creator of lift from a wing. Thrust plays a part on holding up a plane if the aircraft is flying nose up such as in a climb, or a nose high landing approach, then the thrust vector is inclined upward and it provides some of the upward force but that is directly from the engine itself, that is upward thrust, not wing lift.

My point about the F-104 and the glider is that in level flight both rely on their wings to provide the lift, not some other object like the engine. Clearly they both develop different amounts of lift, each must produce the same as their weight and the F-104 can't produce enough at 30kts, but that doesn't alter the fact that when flying in level flight regardless of speed, or weight, or thrust, a plane uses its wing to fly and not engine thrust. Very few planes have more thrust than weight and would simply find it impossible to be airborne on thrust.

As any pilot knows, lift is created by money. The more money you have, the bigger the plane you can fly!

H