The A/C avoids icing by cycling the compressor before it gets that cold. Get a thermometer and place it in the center duct. Somewhere in the forties the temperature should stop and go up a degree or two before dropping again. You may not see this with a regular thermometer but a 2-4 deg swing will be easily seen with a digital thermometer.
If the temp continues to drop into the twenties before heading up, up, up, then you have a freezing up condition.
I do not think an icing condition would be as apparant in air volume as it would in air temp. When the air volume is down check the defrost ducts (and heater ducts) to be sure that the air hasn't just been diverted.
Bosch Master, ASE Master, L1
33 years MB technician