Thank you for sharing your AI file.
What is questioning me when I see the curve you created in Illustrator is why are there so many inflection points in it.
This kind of piecewise curve is tricky to manipulate with Rhino.
I don’t know it it may help you, if it is compatible with your design intent or with your workflow but you may try to use the curvature tool instead of using the pen tool.
This tool helps to simplify drawing paths and allows to mimic the type of curves you are creating with the pen tool quite easily.
I did a quick test and observed that using this type of curve in Rhino leads open surfaces instead of polysurfaces.
Implicitly it may help you to get better results with fthe FilletEdge command…
Just an idea to improve your workflow and stick to Illustrator for 2D as expressed.
Hope it helps.
Regards
Rodolfo Santos.
