I think u had the right idea, there's no wrong way to do this just what works for you. You need recirculating air no matter(unless u use co2) what so u should have laughed at him. You may not need to vent the hood if u need heat. so u can take the ducting off on cold cold nights. U might as well set up your ventilation in one strait shot. You would use separate ducting for the hood if u were really fighting high temps + venting for the room w/ a scrubber.

I would do this, connect the venting from the hood to a inline, then to a scrubber in the attic. Is the attic sealed off from the garage? As for intake its up to u. where ever will keep the room cool but not to cold.