We already suffered from 2 of the 3 years this new Rheem system has been running, due to low airflow. It eventual caused so much vibration at the compressor that is shook all the outdoor unit's anchor bolts out of the tapped holes in the anchors ... all but one of the four bolt were dancing on the floor of the outdoor unit. I used the excellent install instructions, and determined that when they installed my Mother's old simple analog thermostat after the 1st year of operation, they forgot to change the dip switches on the air handler, so she never ran on the high speed for 2 years, and was constanty beating on the compressor.
There's no reason to change an entire evaporator coil if it's not leaking. If there's a restriction it will likely be in the metering device just prior to the evaporator, since it is the smallest portion of the entire refrigerant loop other than the driers, which are actually filters with desiccant included.
If by some miracle the parade of techs cycling through your mother's system get the refrigeration portion of her system working perfectly, and the evaporator STILL freezes up, then that's an airflow problem, period. Getting that resolved will mean finding somebody who understands duct design and layout, who is a person as rare as hen's teeth, it seems.