Firstly I'd get the a/c problem sorted. The a/c does a good job of dehydrating the air within the cabin and really does stop the windows misting up. I leave my climate controls set to auto everything with a/c on.
I'm not 100% on this but IIRC when removing the pollen (cabin) filter one must have the re-circulation flap set in the open(?) position, otherwise one can cause it to fault and not work correctly. Hopefully someone more knowledgeable will clarify that last bit!