As far as I know if you want a very neat installation without any wire exposed, you can only do it under the diffusor. There is no place to hide the wire completely on the rear glass.
Also make sure your rear cam has a very high refresh rate (you can tell by checking if it uses a fiber optic...
Yes
No, I am currently in China. The unit costs 500 USD.
It is possible, but the routing of the fiber optic wire might not be as neat as running through the rear wheel arch to the cabin. Also installing under the diffusor got you a very nice view and coverage of the rear car when you go...