Until there is an official meter, there should not be a cap.
We pay a flat rate for this service, differentiated by speeds on price.
Comcast is perpetuating underutilization of their service by not clarifying the issues you raised.
I can't imagine why they'd want to exclude control/maintenance packets from the cap. I'd be honestly surprised if they did. And if they did, you'd have to make sure that your third party meter treats data the same way.
Comcast is playing loose with this, but the truth is this: We are nearly halfway through august and still no meter.
Until there is an official meter, there should not be a cap.