The problem(s) I see with what you are trying to do, is that you're trying to cram too much railroad in to the space you have.
You have said you want 2 mainline runs... how about 2 levels? An upper and lower? How important is the turntable to you? They eat up a huge amount of real-estate on a small layout. A small engine servicing area with 2 or 3 tracks side-by-side is better. Do you want any scenery? Because with what you have, you're only going to be able to squeeze in a little grass between the tracks.
Do you just want to run trains, or do you want to do some switching? Have you ever built a layout before? The more complicated the track layout is, and the more turnouts you have, the more problems you are going to have if this is your first.
You really would be better off thinking about what you really want, and design a simple plan that will be easy to build and maintain. Then, once you've built it, run it for awhile, and added some details, you'll have a better idea of what you like and don't like in a layout, and you'll have a good idea how to build the next one.