Hi Grant,
After I cut the first window with the knife tool, it all seems fine. Ngon is on flat surface of course. It all looks fine, but when I tab into object mode the window cut is not visible anymore.
If I remove the Face of the Ngon and leave a hole, then I can see the window hole cut into the face of the triangle. But I wonder if something is wrong because in the video when you tab into object mode, the window is still there.