Mazda 3 2012 Problems

You need to measure the idle speed and compare it to the idle speed spec. You can do that using a tachometer. You can check the fuel economy over a tank full of fuel and compare it to what you should expect. Mazda's official claim for the Neo was 8.9 L/100 km and I would expect your car to be around that mark.
 

The feature is only really effective in heavy stop-start traffic where you can save fuel by not idling the engine while you're stopped. If the traffic isn't stopping and starting regularly the system doesn't have much impact on the economy. I would expect carmakers to beef up the starting system to ensure the more frequent use of components like starter motors doesn't lead to their premature demise. I would also expect them to have thoroughly durability tested cars with the feature to ensure there are no hidden issues of the sort you raise. On the cars and trucks I have driven with the feature I have always been concerned about the restarting when you need to move off, and you would want any delay to be minimal. The "gung-ho racing driver" behind you has the responsibility to avoid running into you, but I understand your concern.

I would write a letter to the dealer principal setting out your complaints about their service and send a copy of the letter to Mazda. Follow it up by asking to meet the dealer principal and explain your dissatisfaction with their service to him face to face.