From your description, its the front seal of the Air Conditioning (AC) compressor that is leaking. And its not really clear on what causes that to leak, except excess shaft / internal bearing wear. With that said, you could evacuate the system, pull the compressor, remove the A/C clutch, remove the seal and re-install a new one, but my guess it would be much less hassle (and likely cost) to just replace the entire A/C compressor.
The A/C clutch has nothing to do with sealing refrigerant at all. It only controls the ability to spin the compressor shaft or not, via a simple electro-magnetic pressure plate clutch.
And as for the temperature blend door inside the passenger compartment, a simple test to verify it is shut is to measure the temperature of vent air (A/C off) at full cool setting, while car is moving, and after the car is warmed up for a few minutes. Generally you would see a temperature of 5 to 7 degrees F over outside temperature, depending on your vehicle speed. The extra heat comes from the engine compartment heating things up a bit. Best tool for that job is one of these:
That tool is also handy for solving arguments about thermostat settings in the house. Pays for itself in days!