That slow flap problem would most likely be vacuum related as the soleniods that power these are all vacuum controlled. My guess is that the problem with airflow is the same issue.
If one "door" is not opening fully it might cause the outside air to not get through to the heater box hence no airflow in the heater mode.
It took me about three days to actually figure out what all the vent doors opening and closing in combination did inside my old 115 but once I figured that out and then fixed the vacuum leaks it worked great.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique