Intake and exhaust fans depends on your setup. Using CO2? You want them closed. Otherwise, i let the air flow. Just make sure you have a carbon filter on the exhaust. I personally like using a HEPA filter on the intake as well. Call me paranoid.
Keep all the fans on all the time, it takes 5 minutes for plants to use the air around them, so its always good to have new air on your leaves all the time
Sorry dont agree, its ok for the fans to be off when the lights are off, if your fans are on all the time the plants will put energy into stems for a bit, but by giving them a break when the lights are off the plant can also focus on what we grow for, the buds. again its all IMO.