Roman Tub Faucet Removal
Yes, the design is normal. Yes, you loosen that allen key and the spout pops off. No, it is not top installed. If you want to remove it you need to access the bottom somehow be it opening tile or removing the tub. Is it a jacuzzi tub? If so their should be an access panel for the motor, can you reach the faucet through that?
Why do you want to replace the faucet? If you are just tired of the way it looks, the finish is wearing, or it's leaking you may have another option. If you have a good name brand faucet you can usually buy new trim for it (handles, spout, and escutions). The trim as well as the cartridges can all be changes from the top.