The way you sent the first STL file worked. Use 7-Zip or Winzip to compress the STL file into a .zip file. Put that on your Google Drive and reply with the link. That is a good way to send me the STL file. Do you need another way?
The image you show has only one piece of geometry and next to it is the facet body. Put the skin on that facet body and then look at the two edges of the two geometry squares with the facets turned off. The facet edges are identical for the two surfaces. If you click the same points for each surface, that will result in smaller deviations on the mating edges of the spline fit geometry.
YOUR PLAN versus MY PLAN
The structured mesh video describes my plan, which is to create a block of air and slice it off at the wing surface. Each block of air above the wing or below the wing can be hex meshed. My plan directly meshes blocks of air and at no time is a filled solid of the wing created. You have a different plan which is to create a filled solid body of the wing. I don’t like your plan because it does not address how to mesh the air, which is the end goal.
You may have trouble understanding the difficulty of meshing the block of air that contains the trailing edge becuase you are not thinking about the air, you are thinking about the wing. Follow my plan and put a plane 5 mm before the trailing edge and another plane that is 5 mm past the trailing edge. Cut out that block of air. There will be a wedge shaped piece of trailing edge cutout of that block of air, but you have to extend the sharp edge of that wedge from the middle of the block to the back side so you can end up with a hex meshable top block of air and bottom block of air. You need to create the surface that is the orange line in the illustration below to split the block into two pieces. Recall that the trailing edge is not a single straight line, but a lot of short lines which makes the task more difficult.