Creating negative pressure, I will look into this. Considering using like black tarp-like material to cover while inside of the closet and then putting up the shiny reflective material in the lower region.
If you "top" your plants it will help keep them lower and fill out more, sounds like you are figuring it out. I have heard of people binding the main stem so it grows sideways? This seems like a cool tactic.
Sounds like fun. Enjoy and let the plants do their thing! You may have to alter the height of your lamp depending on if your light is for 2 ft from canopy vs 5 ft from canopy, a shower rod is an excellent way to make the light adjustable.