Actually, for most practical uses, you need two Smart Radios, one for the brain, the other for the controller.
For programming (autonomous-only mode), Smart Radio in the brain is enough, but then, the controller won’t connect using the gray radio, so you won’t be able to use the controller without switching back and forth. Thus, you’d better switch both brain and the controller to the Smart Radio, then you can both program the robot and use the controller easily.
As for USB, you won’t need it for programming from iPad, but you may still need USB cable for firmware updates.
In theory, iPad can be a controller with a proper app. I don’t have iPad so I have no idea if there is an app readily available for that, but there is a public SDK and the controller protocol is fully documented. There is even a simple demo application source code posted. But the real controller is so much better, which makes it a moot point I think.