I'll try to answer your questions:

For question 1:
Eva foam varies in flexibility. it depends on what your term of flexible is. EVA foam is rigid, but will flex under pressure. Most EVA foam pads come with criss-crossing raised edges on the back. Some might be small, most are large. These criss-crossing edges give backing to the EVA foam, and if one was to use a heat gun and pressure to level these bumps, EVA foam becomes very much more flexible. If you want to use this technique (for tight bends and such) it's better to look into EVA foam with smaller bumps on the back.

For question 2:
The main reason you re-enforce pepakura is because it is paper and very thin. Since EVA foam tends to be at a thickness of .5 inches ( a centimeter) it doesn't need much re-enforcing. If you do plan on making foam armor, you are going to want the little bit of bend that EVA foam has to offer. Also when (and if) you decide to use plasti-dip or a similar product on the EVA foam (it makes the paint stick easier and longer/prevents cracking) it will make the foam a tad bit stiffer, but not by much.

For question 3:
EVA foam is very durable, but the durability of the armor depends on how much time you take in making it. If you decide to just use the EVA foam glued together (hot-glue works magic) than the project will be very durable. If you decide to brush internal creases and folds with melted hot glue, it will add to the rigidity as well as prevent tearing of the foam. Tearing of the foam is most likely to happen when high amounts of stress are placed onto a part that is not meant to bend or stretch more than slightly. If you are making EVA foam, it's always best to make the high-traffic parts (much wear from movement) with seams that are clamped with clips (parachute buckles are your best bet).

The bonuses of making your armor out of foam is the increased durability and strength of your armor, but keep in mind that details may be lost in pep since tiny pieces must be custom cut or made from "foamies" (thing sheets of craft foam). Hope I answered your questions, and PM me if you have any more.

Thanks! Thats what i hoped to hear about EVA foam