to answer your first question... u have more then enough airflow..a basic airflow rule of thumb is.. you want the air exchanged twice a minute.. your working with about 45 cubic feet of space.. and a 495 cfm fan you will be replacing your air about 10 times a min..

secondly i would use the fan to pull air through the carbon scrubber rather then push it it will work more efficiently

yes the exhaust should be towards the top because heat rises..

and just make the passive intake hole the same size or a little bigger then the exhaust hole.. also dont forget to light proof.. can be done with a simple dryer exhaust vent...
best of luck man..