RE: Venting battery box?
I would think passive venting should be fine.* Getting the batts out of the engine room will do wonders for the temperature, so I wouldn't worry about the minor difference you'd get with a fan.* Main thing I'd think you'd want to do is to keep the odor and corrosiveness of the battery gas out of the cabin.
Goes without saying that, if you do decide to use a fan, use a brushless one (i.e. computer fan)*as sparking brushes in a hydrogen-oxygen atmosphere probably isn't a real good plan.