Working with non rectilinear surface PART II

Hi all,  I posted a problem yesterday (http://www.grasshopper3d.com/forum/topics/working-with-non-rectilinear) and have had some progress, but still have a few issues.

My goal is to have a surface with a "courtyard" cut away from the middle. The attached screenshot shows my base geometry: one layer is a mesh of the desired floor slab of my building with the courtyard subtracted, the other layer is an isotrim surface which is basically ignoring the trimmed courtyard geometry of the slab.  I ultimately want to diagrid or panelize the slab, which I wont be able to do if I cant get rid of the courtyard.

My question is probably based on a misunderstanding of a fundamental rule but...can I generate a surface (as opposed to Brep or mesh) with a "courtyard" cut in the middle?  My logic in the attached definition allows me to trim the surface to create the courtyard, but then ignores that subtraction when it comes to surface curvature analysis and isotrim divisions.

Do I need to make a step which merges the projected curve with the original surface so that the subtracted courtyard is "forgotten" (for lack of better description)?  Or does isotrim always ignore projected and trimmed geometry?

Any insight into this concept or my definition in particular would be appreciated.

Best Regards, Matthew