IMHO, this could only really work with like a screen of green because that is just not enough space. In a normal tree grow, for you to veg them till 18 inches, they would have out grown your 8" length before you have even started to flower. Even with something like a scrog, your training would have to be impecible.
You have to remember that MJ is a very heavy branching plant. It usually does not grow straight up and down, it also branches out, looking for light.
The strains you have chosen will not work in a situation like this. Big bud is a commercial plant, that gets very big with heavy side branching, white widow is not a small plant either. You would need a very indica dominante plant for a scrog, and if you are not willing to do that, then you would need something like Low ryder.
You would not want to grow anything that can get bigger then 3 feet, at the very most. My advice would be to check out someone like Brother's Grunt's grow journal. Have you ever thought about building a grow cabinet of some sort? check out the grow journals section and see how some other people grow with comprimised spacing, because some of the ideas our fellow members come up with are very ingenius. Space is not really a problem for me so I can not help you with the construction, but I bet you could learn a lot from others in the grow journals.
The only other option is to straight up find a new place to grow, or grow outside. I know this is not what you wanted to hear, but I did not want you to get your hopes up, but THESE ARE ONLY MY OPINIONS.
Either way good luck and keep us posted on what happens.