It never was backfire. It's afterfire, there are youtube videos explaining it. The noise comes when you let off the gas and it's over rich. It seems to happen around 3k for your car. I wouldn't worry about it, it's common on all older cars. The soot issue sounds like something better to chase since it's a california car. Basic maintenance, plugs, oil, o2 sensors. Also 2012 probably falls into the weak piston rings recall, has the car been consuming oil?
Going back to what other shops said, what tests have they done to confirm this. Backpressure, did they drill a hole before cat and measure? Valve timing, take off timing covers and match timing marks.