Greenhouse Fan and Vent System

You'll need another contactor -- one for the fan and one for the vent. If you have never wired a thermostatically-controlled system before, I would suggest that you go to see a HVAC contractor and ask, "What in the heck do I do with all these wires?" The "Auto" position on the switches connects to the thermostat terminals which closes the connection to a (probably 24 VAC, meaning your contactor coil needs to be 24 VAC also) control transformer which is either contained inside the combination contactor enclosure or will have to be supplied by you (and you only need one). One of the "Auto" contacts will connect to the lower temperature terminal for vent operation, and one "Auto" contact will connect to the upper temperature terminal for fan operation. The "Hand" position switch contact on each of the control boxes supplies 24 VAC directly to the contactor, thereby closing the 120 VAC connection for vent and fan operation. There may be connections for the "Off" position, but they're not needed.

Yes you can set a control just be sure to buy one that is sealed from moisture and has water tight connections.

im not sure how your system is set up, need a wireing diag, but iv done something similar,, useing a 3 way sw, and use one leg to the original auto controls, and the other leg to direct control of fan,, hope it helps,,

You indicated a desire for independent operation, did that mean four different ops?(2 vents 2 fans). Manual overide would be as simple as a single pole switch bypassing each pole pair of your contactors. Then using a Low voltage PS and matching Low Voltage coils with your thermostat for your automatic control. Considering the humidity, I would agree that contactors and switches be housed in weather resistant boxes with water tight wiring ports. Additionally the bypass I suggested provides a redundant backup incase of PS, T'stat, Coil, and or contact failure.