Hi gizmo,

re: shell with fault - if you zoom in you can see that the pieces did not get really aligned properly with each other:








The pieces could join because some parts of them were close enough, but other parts were not close enough so did not get joined into a single shared edge.

You'll need to get a better alignment than that to get a clean join.

But after that I still would not be surprised if you could not fillet it, because you've got those degenerate edges in the surface patch - that's where your 4-edged surface has a tangent edges on its surface corner instead of a distinct corner, that causes problems with offset operations and part of filleting involves an offset calculation.

Again if you would use the technique described earlier on building a larger surface then trimming it back you are more likely to avoid many of the issues you are running across, you are attempting to do things in a more difficult and low level way right off the bat and that is increasing the difficulty level for you by quite a bit.

- Michael