Hi, this is a thorny issue as it could be any number of things. But, my first thought is that the back mounting surface is not sitting flat on the hub. Over-torquing should not cause cracking, unless the green hulk torqued them.
Something just does not sound right if all four wheels were not holding their torque settings. Maybe the studs are binding on the holes, maybe the holes are not perfectly spaced, maybe the studs are stretched, maybe..., maybe...
The penetrant will probably change the torque setting, though not all brands/types do or would. Best idea for that is to clean all the studs with brake cleaner and then add a faint smear of anti-seize.
Using an air-wrench is not an issue if they are using a pre-torque extension that will only let them toque to a fixed load, usually about 10-lbs below the hand torque setting. They are used to get all the studs tensioned to a known (roughly) place.
However, I get the feeling you are looking for tangible evidence for a claim and it would be foolish to go by anything you are told on a forum of faceless souls, (me included) no matter how well intentions they may be. If it is claim material you are after then you need to get an Engineer and get them to make a report on what they find.