You can leave in the rocks/pebbles/sticks if you boiled then baked them like you said, but not the dirt/plants
No! You don't know where they have been and what they have come in contact with.
Yeah that's fine.
Look, just because they live in melbourne doesn't mean you can get a few things out of your backyard and think it's okay.
You need to buy coco peat for substrate and some plants. You can get mondo grass for $2 a punnet at bunnings and that is great for them.
You shouldn't be 'Thinking' of buying them, you should be getting them. Wild insects are not good for him.
They live for as long as you can keep them alive but keep in mind they will grow.
Also with the water you have to add a dechlorinator to it. Tap water is no good for the frog and it will kill him.