To unbend: I've used your file and represented the circular neck as a polygon. I arbitrarily went with 16 sides as a tradeoff, you can get more fussy and edit the perimeter of the polygon to close to that of the circle. The first picture inset shows the polygon coned, the big picture is both profiles coned and added. Then I format the solid as a surface and remove the top and base with facet edit, removing the internal cavity's faces at the same time. (It occurs to me now that facet edit, highlight external faces and format|surface|from faces will do it faster.) If you leave the internal cavity faces there, they'll interfere with filleting and shelling. The reason I don't shell and omit faces is because TC does surface booleans anyway, and there are less faces to have to work with at this point. Next step is to cut out those vertices of the solid that have more than three faces intersecting ie the quadrant points of the circle and the corners of the rectangle. Those intersections will stall filleting and shelling if something's been overlooked. I've done it using spheres. I've locked diameter at 20mm in the inspector bar and placed them at the multiple-face intersections. 20mm because the cutaways leave the intersection points that come with filleting and shelling well clear. The spheres are subtracted from the transition, and because it's a surface, that process doesn't create more faces. Then I've filleted all of the longitudinal edges 5mm, to leave room to make sure that I don't produce more multiple-face intersections. Then I've shelled the transition 2mm and booleaned out a .005 thick slice so that the surface is discontinuous, after which it'll unbend. If you look at the cutouts that the spheres leave in the unbent piece, you'll see that they're multiple arcs. Looking at the last picture, you'll see how I've continued green lines drawn through the face vertices in the cutouts to their complementaries on the opposite edge. Those are the bend centrelines that TC omits, but this method does make them quite easy to define without special consideration.