I am trying to make some advanced duct work for a special application. It requires some very odd section changes, across some fairly intense curvatures. How I've done this type of work in the past, is by making a loft or boundary boss, then using the shell function. Later I can cut it apart and add construction details.
For this particular duct to work, I need a sharp point to diverge into two square corners. The highlighted blue line is one of the problem curves, the other is to the bottom left.
The initial section is on the right, involving two curves and two straight lines. It must then end in a rectangle.
The part file is attached, although I'm not sure if Sketch1 and 2 will be defined, because they are taken out of an assembly.
These are the top and side views respectively.