I'm a bit confused about your setup... you said you're pulling air into tent from inside the closet, then exhausting outside closet into the room itself, correct? How much extra room is there in the closet besides the tent? Is this big closet, small tent, both, or what? Unless it's a truly huge closet then it seems likely the 600w is warming up the whole closet, hence you're using hot air to try and cool your light.
If my assumptions are correct, then you gotta get cool air into your hood. Go wherever you got to to get it. I feel you on the stealth thing. If you're in a house with a post and joist foundation (meaning you've got a crawlspace underneath) just cut through the floor and pull your air from under the house. If that's not happening you'll have to go to another room. I've got a similar issue (and a cement slab foundation) so I'm pulling air in from my bathroom. I used a dryer vent cover
like this one from home depot and it looks just like any other bathroom vent. You could use something like that or a furnace/ac register. Assuming you're handy, it should fit right in and no one will notice anything special. If you're not handy, go back to buying your weed
A Couple Other Things
In your pics I saw a lot of bends in your ducting, especially for such a small space. Get rid of as many of those as possible. It's not always doable, but I've found it's the single best way to increase airflow -- it's better to have a significantly longer straight run than a short run with a 90 degree bend in it.
+1 on the suggestion about isolating your light cooling from your room air exchange. You've got two fans in a tiny closet... you've got plenty of fan to do it. I'd run the big one to cool your light, and the smaller one for air exchange/filter. Of course this means totally tearing up your venting and redoing it, but that's the job...
Don't bother using your clip-on fans for cooling...if you're doing that you've already lost. Use those fans for airflow around your plants to make 'em stronger and prevent mold issues.
Oh yeah, and don't have nosy visitors. Tell 'em to get fucked.
HTH