I think we are may be seeing 2 or 3 separate complaints here. Does it seem like it takes too long to get heat initially? You’re comment about “15 minutes of driving” seems to state that.
•Heat takes too long – Verify coolant temp to the evaporator core. Ensure both hoses are hot and within a couple of degrees of each other. Coolant sealer products have a tendency to plug up heater core passages. While this would affect air temp, it would not affect air volume.
•Air volume is low – Remove blower fan assembly and ensure motor and housing are free of debris. If possible, attempt to inspect evaporator core and heater core from blower housing. The core(s) may be restricted resulting in a reduced flow.
•Heat only on the driver’s side – This may be an issue with that centralized flap, or it could be the result of the low air volume. I would suggest addressing/fixing the total low air volume prior to chasing heat only on one side. If the central flap were problematic, only allowing hot air to come from the driver’s side, I’d expect to see an abundance of air volume.
Do you use the ECO button on the car? In Eco mode the PTC heater grids are turned off to save energy. (poor heat condition).
Have you located any obstructions in the heater box?
That main actuator door may be your cause of both conditions. Removing the actuator and marking its sweep and verify the door moves freely is the only way I have ever tested a blend door/actuator concern.