Mazda 3 Problems

You could take your complaint directly to Mazda's customer relations people. You might get a hearing because the warranty had only just expired.

The knock could be caused by any number of things and the dealer is best placed to locate the problem. I would persist with the dealer, explain to him that you are frustrated and want it fixed, and keep on his back until it is.

Most cars today have a firmish ride and there's not a lot you can do about it. You could perhaps ask your dealer if you could drive another Mazda3 to compare with yours and make sure yours is representative of the model. Assuming it is you could ask about different tyres, even different shock absorbers. You could also consult a suspension specialist, such as Pedders, for their advice on what you might do.

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.
 

Maintain the rage at your dealership, this issue is clearly a dangerous one and needs to be fixed. If the dealer is showing no signs of being interested in fixing it go direct to Mazda and demand their intervention in sorting the problem out. I would expect Mazda to fix your car without charge given that you have reported and had the dealer check it while it was under warranty.

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.
 

First check for anything that might have come loose under the car and knocking against another component. If there's nothing you can see then have the shock absorbers and the mountings checked.

When doing your research you need to dig deeper and find out the claimed fuel consumption for city and country driving. The Mazda 3 manual is claimed to return on average 7.9 L/100 km, but when you look at the claims for city and country you find that it does 10.8 L/100 km and 6.2 L/100 km respectively, which lines up reasonably well with your experience remembering that the claimed figures are not real life ones like your own are.