The key to "dusting" and "fading" with the airbrush really doesn't have much to do with making the paint "thinner". The result will be just the opposite, as you have pointed out. The key is keeping the airbrush at about 6 to 8 inches from the model and keep it MOVING. Make several passes at the extended range, and I too would use an acrylic, such as Tamiya buff. Move the brush over the work in fast swipes, applying layers until you get the level of fading that you want. Since you are painting in a less than ideal environment, why not give up on the airbrush altogether and go for MIG pigments, they are designed just for the purpose you desire and are great products, you can apply all the dust you want with an old paintbrush.
Like this sprayed with a "buff" acrylic mixture at about 15 psi.;