Trainiac - with regards to the width of modules 3, 4, 12 and 13, you will be very disappointed if you build them this narrow. What you'll quickly find is that they are essentially limited to having just running through-tracks on the them, as they are simply too shallow in depth to create either reasonably realistic scenery, or potention industries/switching on. Believe me, there is nothing less realistic looking in model railroading than a far too narrow shelf.
In addition, as you already appear to summize, your corners will need additional attachments to accommodate any reasonable curves. Narrowing your center island and widening your around the wall modules would be the best approach.
And as someone noted upstream, the widths of you aisles is going to make it a rather tight fit for the operator. I have two aisles that are a bit over two feet wide and find them too narrow for my average-sized body to move around in without potentionally contacting something on the layout's edge with my elbows when turning around.
CNJ999