They may not end stacked with other HQs. Independent artillery can only be supplied by Corps HQs. Artillery can trace supply to a HQ of a friendly nation in the scenarios but not in the campaign. Operation Cobra III.B.2. Units with factors of zero do count for CAM purposes.
Retreats are still likely, of course.
Allocate air interdiction markers. Place cut markers. The Fall of Cherbourg IV.B.6. Coastal artillery units in the same hex as a fort are always considered to be outside the fort and only count towards total stacking limit of the hex not towards the fort's. An unoccupied minor fort or fortified area is destroyed when occupied by Allied units who may continue to move and do not pay any additional movement points.