3. It is 21x1.5x3=94.5

Not the other number.

So your fan is going to rock in there. If you haven't purchased the fan you you could easily step down to a smaller fan. Or get the larger fan if you think you may expand in the future. Either way you definitely want a fan controller. Just note, some fans are quite loud. Smaller fans make less noise. Do more research if this is going to be an issue for you. I can recommend some quiet fans if needed.

You can turn fans off with lights out so long as odor is not a major issue. They will start to stink up the place by the time lights come back on. You can experiment. Also, odor becomes an issue once you hit flower although some strains start to smell in veg.