Very good point about allowing for the paper thickness. Is there a reference table anywhere that lists the dimensional thickness in terms of paper weight?
It is important to know that paper weight does not equal paper thickness. Of course heavier paper can be thicker than normal paper but depending on the manufacturer there might be differences. The easiest way to find out how thick one sheet of paper is this: You take a number of sheets, count how many of them there are, put them on top of each other to form one solid paper block, measure the thickness of the block and divide it by the number of sheets used. That gives you the thickness of one page. Another way would be using calipers.
Also how much extra backing do I need to add in order to get a good effect in 1:400 scale? I'm thinking I might also apply this same technique on the double-deck covered roadway along the front of the terminal itself (which faces the parking garage across the arrivals roadway).
That depends on your personal preference. I would make a small mockup to check everything out. If you are pleased with it, go with it. If not, make some changes till it looks good. There is no one and only way to do it, there is no right and no wrong. All that matters is your decision how complex you want to go with it and your personal preference.
Something I noticed is that the curves on the original consist of a series of straight bars which are assembled in a row at a constant angle. This is exactly the way a 3D program forms curves. You could make advantage of it. Those tiny edges may be used as natural folding / cutting lines on the model:
The red lines represent the edges forming the curve. Everything with the same colour is a straight face. This little trick can be used to acurately place the coloumns, too. They should be on each red line.
Would I necessarily need to go through the trouble of making a separate Sketchup model for each layer of material or could I get by with making duplicates of the face shapes in Inkscape then trimming away the edges to allow for the thickness of the overlying layer(s)?
Again, personal preference. Since you are dealing mostly with straight faces you can edit the unfold in InkScape later. Using SketchUp might be easier since you don't have to fiddle with the lines later. But that is just my opinion. Yours may be different.
Just make a small mockup and toy with it to see what is best for you.
BTW, just in case you haven't got it yet, you're gonna need THIS:
Sketchup Plugin for Unfolding or Flattening Models - Works in Sketchup 2016 - skotekar/flattery
github.com
www.blender.org
