In your second model, you want to look at this area:




Zoom in there to see this:



That area there where you have the 2 edges so close together and intersecting with the other objects seam edge is going to be problematic for many operations down the road. You may want to revisit the method you created that surface with and realize how that little area was created, then avoid doing that in the future. Most likly when you did the extrude or whatever op you used top create that surface you had 2 seam points just grazing by each other.

But you can get what you want with your first initial method and Merge.