I've done several of these terrariums, and hopefully I can give some input.
Yes, anything that you use as a substrate will degrade quickly. I've tried several mixes, and I've usually had good luck with growing mosses, ferns, and bromeliads in the ground up coconut soils (i.e. Eco-Earth sold as reptile bedding). If I were doing carnivores in there, I would probably move to more of a peat/tree fern fiber mix, to keep it airy. I'd imagine you can grow plenty of other things in that mix, too.
As for the background, the foam works alright. Don't bother trying to push the rock into it to get shapes, because as the foam expands, it won't matter. wait until it expands, and then tear at it with your fingers...you'll get plenty of variety and random changes in the background that way. It works well, but unless you have a misting system, it does like to dry rather quickly on me. If you don't like the epi-web, you can also look into tree fern panels for the back. Cork bark also works well, just try to keep it from the water line. If you let it dry between sprayings, it should last quite a few years.
And yes, you can remove that center brace. I usually do it with a Dremel.
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