In your troubleshooting so far, you have confirmed that the evaporator core is clean, and that you have a good air flow when the defroster or floor vents are selected. You also had an auto parts store check for a vacuum leak on the actuator door that switches between vents and defroster. They found no leak. You also mentioned that there was a few small dents in the fins of the evaporator, but it was believed that was not causing the issue, as the air flow is great coming through the vents or the floor, all of which comes through the A/C evaporator.
At this point, I would check for air leaks in the duct work, the only other thing would be to physically confirm the vent flapper door is working properly.
I don't know how different of an answer you will get here, but remember we can only help you as much as you can help yourself, and there was still some questions that were answered. What is the flapper door doing, and is there an air leak some where?