A combo of my file and your new one,
There are 3 sketches in the top of my file, they were not in the new file.
Your new surface body was used as the cutter for Body 19, but you had that body removed, restore that body so you can split body 19 with the surface, it adds a new body (24), and now you can remove body 19 and 21.

Sketch 13 was not in the original file. Sketch 15 is causing trouble the curve is not tangent to the surface body edge, and needs to be.

My sketch 16 is on an offset plane, has the surface body edge and the tail point, projected to it, those two articles are joined with a tangent spline. The tail body is cut with this new curve.
Then there is a new sketch same as you did with the top outside triangle, Make it a 3d sketch, and use Project > Include 3d Geometry, select the point of the tail for the loft.
Start a Loft Cut, take the top outside triangle, to the point as profiles, and then select each (3) Rails (body edge) of the triangle.
Hide / Remove what you don't need, and Combine Join what you do need.
Might help....