1. The three ideas you listed sound great but it is critical that if you use soil that it be organic.
2. An inch, these frogs don't borrow but deeper, as deep as needed for area with the plants.
3. They will probably be eaten a bit but not enough to destroy the population. Springtails and woodlice like to borrow and hide and your frogs will spend most of their time climbing. You will just have to worry about over feeding your frogs.
4. Ohh Kurt where are you?