Hello and welcome to FF! If expanding foam is pond safe; it will be OK with frogs. You can silicone the background in, or see if can locate Titebond III (waterproof when dry) in local market and use that instead of silicone.

Boiling or baking beach rocks or driftwood is fine. Would not collect anything from an area that has pollution or similar problems. You do not need to remove the bark with most frogs. Good luck !