I think putting the exhaust fan opposite you when standing at the sink with the fan at sink top level is a perfect start. I cant find the Kodak book on building a darkroom right now so someone else step in. I think the minimum figure they gave was that you should change the air in the room every ten minutes or six times an hour. I am not familiar with a muffin fan. It sounds bite size, but I dont know. You need to calculate the cube of the room and determine how much air the fan will exhaust in that position with the bends you mentioned. I personally feel a good strong breeze is life insurance and have added fans accordingly. I value the silence of the Panasonic Whisper fans. I either want to listen to music or think undisturbed.
Ideally you want a sealed dark room pressurized by your intake and relieved by your exhaust. If as you say air is coming in from all sorts of places, those places have probably been there a while and dust will arrive with your air. That defeats all the reasons you built the darkroom in the first place. Another consideration of those openings is that light may be fighting to get in with the dust and air. After you have gotten tired and hot building the darkroom, a good cool off is to turn on all the lights outside the darkroom and above if this is in the basement. Close the door, turn out the darkroom lights, sit down and wait five minutes to adjust your eyes. Then move around, up, down, all around to look for light. If you see any you probably arent seeing the dust that will come in with it. If you can see where you are going there is light in the room. When I finished building my darkroom I did this and then spent some time caulking places I had missed. The payoff on dust comes when you do a series of twenty enlarged prints that dont need spotting. I print mostly 16x20.
Just a side note, I mentioned in the earlier post that my fans are in the rafters and I use 4 PVC to get down to the sink level. I would prefer to go directly through the wall, but my darkroom is in the basement, below ground level. The other side of the sink wall is about four feet below ground level.
Hope this helps. If not please ask.
John Powers