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The temperature issue sounds like a problem with the blend door. The system likely has a coolant shutoff that blocks coolant flow to the heater core in the MAX position.
If the air blows out of the proper vents on each setting, your poor airflow is likely caused by an externally plugged evaporator (dirt and debris).
Check to see if the air always blows from the defrost ducts. If so, this would indicate a vacuum issue.