I'm trying to create a hollow handle from the wire frame in figure 1, with a 3.15mm wall thickness, and without creating a degenerate surface. I can do the top portion fine (figure 2) but I cannot get the bottom half to behave. A boundary surface creates a degenerate surface (figure 3, note the converging lines on the left), a surface fill creates a chaotic surface (figure 4), and a boundary surface without the tips makes a non-degenerate surface but can't be thickened. I've previously used some hackey workarounds, but these all make life very difficult in the late stages of making the handle.
What is the secret sauce to success that I am missing, or the fundamental reason that this isn't possible? Attached is the original file.