Some auto makers use dual injection, similar to what you described. It's actually quite effective at eliminating the main issue with gasoline direct injection. Unfortunately it doesn't sound like this new engine will be dual injection.
I'm not familiar with Mazda gasoline direct injected engines. It's great that your wife's Mazda seems to be running fine. But every direct injection engine engine will eventually suffer from carbon build up. This carbon build up isn't like the combustion chamber build up. Revving the engine, high rpm runs, driving it like you stole it, won't help with this issue. The carbon builds up on the back sides of the intake valves since there is no gasoline to wash this area off. With port injection, throttle body injection, or even carburetion, fuel constantly washes the backs of the valves, keeping them clean. The best way to help with this issue is prevention. Installing a catch can help reduce the problems.