I wouldn't do an elastic waistband - it's all going to be pretty heavy. I would do buttons those REALLY heavy-duty snaps or something.
Yes, you would sew them "normally". I'd just put them on top of each other and treat it as one.
For question 2, what really matters is what quality you, personally will be happy with. I'd also consider that the model is really simplistic, and I think the skirt is fuller and the "butt capes" more drape-y than they can make on the model alone. Kind of like this
, not exactly, but that's the idea - that it's not just a "flat" piece of fabric. You can see that they've shaded in some drape shadows and such on the skirt and buttcapes.
For question 3, I'm not quite sure what you are referring to, sorry.
You've drawn the middle gold piece as one solid thing, but on the model it is two sides. Is this what you mean ?