Answers to your questions,
1) for fake wall, you can use cork bark and just place it behind, but because it is not attached to the wall, many a times, insects such as crickets that you feed the frogs may get stuck there, so not very goodd. You have have styrofoam wall which is cut to size and force fitted in so that the water level will not push the foam up. The partition water can be made with many materials, such as slate rocks stack to form the partitio.
2) the soil can be a mixed of expanded clay pellets at the base, then larger pebbles, then a layer of garden compost soil for you to grow the plants or mosses as a top layer. If you want, you can just go with expanded clay and pebbles but that will limit the type of plants that you can plant.
3) the cemented styrofoam normally will be washed down with some sand and PVA glue so that the constant beating of water with be fine.
Do a quick sketch of how you want your scaping is to to do so we can give better comments. Cheers!