My expertise in exporting to dxf format is very limited. What I could see is that the SolidWorks splines are exported as splines in the dxf file. It is obvious that your display/cutting software has the ability to interpret these curves. Isn't facetization simply a way for them to lighten the representation on screen? I think they need to offer an option to manage the graphics quality...
To convince yourself of this, open the dxf file with SolidWorks as a 2D sketch in a part, you will see that the splines are present and not facetized.
One question: rather than undoing the smoothing to keep only the sketch of your couple, you have an option in the dxf export that offers to use a selected face. You just need to build each fuselage section as a separate volume body, and make invisible those that might obscure the face you want to export.
One last one for the road: the use of splines for the guide curves in the smoothing functions would allow you to have tangent connections between the successive sections of your fuselage. Not fundamental for creating an actual mockup, but so much more beautiful in SolidWorks. You can also try a single smoothing of the entire fuselage, and then make cuts through the shots of the couples...
