BNSF, your trying to stuff TOO much into a small space. Double decking wouldn't solve a thing, you still have the issue. Be happy with the around the walls for the time being. Heck when I was your age, I was greatful to find room for 4x8 sheet.
You have the chance to do what we all suggested and use the middle for a yard, thus avoiding the tight clearance. Doesn't matter your age, we are all speaking from experience on this one. If you had any operational experience, it will tell you 2 things
1.) More ISN'T better
2.) Isle space is PRECIOUS
Just because you are young, doesn't mean 18" will work. It just means you don't want to limit yourself down. Sometimes being pulled back by the reigns is a GOOD thing. Learning to start smaller lets you learn the complex nature in track design. You learn what WILL and what WON'T fit into an area.
You seriously don't need to rush into anything.
Start by building some 2'x4'or8' modules out of 1x4's get the benchwork done, then go from there. Narrow the layout over the door entry, since you have to have a duck under, leave it no wider than 8" or so. Less here is a great thing.
If you make your center area a yard and no turnaround loop, then you get isle 24" isle, which while they are TIGHT, they are better then 18". This will give you a YARD and the ability to run trains around, also giving you a yard lead to switch from too.
Take these things you are hearing here and apply them. You are starting to sound like you want an EMPIRE is a shoebox and that won't fly and most will simply lose their patients with you. Just my experience.