CPS staff are highly trained to address the issues of demanding pros such as critical focus. It's like mechanics who are trained to tune race cars vs. mechanics that service a Kia. It's not the same level service. With CPS you can physically talk to someone on the phone and express your concerns.
I think the MACK company are not competent in testing focus. Don't waste your time with them. I would just get the camera back out and test yourself. If there is consistent focus shift try Micro-adjustment. If issue persists send to CPS directly, explain the problem and I'm sure they will either fix or replace.
BTW, as you probably know the MKIII is famous for AF issues, some cameras could never be fixed unfortunately despite multiple calibration/repair attempts by Canon. If you have one of those lemons you're out of luck :(
PS. Here is what I wrote for MA a few years ago
http://arihazeghiphotography.com/MA-web/