It seems to me that most professional growers separate the lights entirely from the venting in the room. They suck into the lights from a duct not located in the room. Suck in from attic and then back out attic for the lights. Venting in the room is for a different reason which is to control odor. SO the only way that will work is if you suck in from your room, through the scrubbers and then out of the room. That's the only way to remove the odor with the equipment you have. Suck from within the room, through the scrubber and then out of the room. If you don't want to run a separate line for just the lights, then suck from the room, through the scrubber, through the lights and then out of the room. This way your lights wont build up stinky odor or cause any other issues. The air will be scrubbed before going through the lights.

The down side to not separating the venting for the lights is that the "cool" air your using to cool the lights is actually hot air that's been sitting in the room. This is why if you want cooler temps, cool the lights with fresh cool air not from within the grow room and then deal with your odor issue separately.