I think the trouble is the use of the term "SubD". What we generally mean is a polygon mesh that has been subdivided (aka "smoothed"). It remains a polygon mesh, just with more polygons and averaged vertex positions. It's not a true subdivision surface. To subdivide your mesh, use the mesh>smooth option, or hit "3" on your…
Split the center edge loop and make them overlap each other.. like a real shirt. Select all polygons and extrude them so they have some thickness to it. This also "fixes" the "flipped polygons" of the collar. Be sure to delete all the polygons that are on the inside of the shirt, as they are just unnecessary polygons.…
Sort of: The point is that these are no splines per se, these are loose edges (not part of a polygon) with a subdivison modifier applied to the object. And yes I connect them via "Make Face" (or "Grid Fill" if its a larger patch). By doing so these edges become just part of the polygon so no additional deletion is needed.…