GREAT question. Applying the glass to both sides does increase your strength. The idea is to replace the paper. The glass with WBPU is more stiff than paper so it provides better rigidity. Doing it to both sides will make your model last longer. Also, larger surface areas will bow and warp with fluctuating heat and temperature. This risks changing the aerodynamics of your model. In dryer climates you may be able to get away with it but its fairly humid here in MD.
You indeed can glass the outside after construction. So, say you were to use standard foam board with the paper on, construct each part, remove the paper on the outside, and then glass. It would work but the paper on the inside will de-laminate from the foam over time. Then you risk the same bowing and warping mentioned above. Not to mention glassing flat surfaces is much easier.
Also, you will want to apply the WBPU + BP mix after a part has been constructed. I don't think I made that clear. So, make your fuselage, tail surfaces, wing(s), and before you assemble them, apply the mix first. It will make sanding much easier. Painting can come later, or not. That is up to you and how much detail you wish to add (panel lines, rivets, etc.)