This is very similar to the question asked here
Here’s the same script applied to your curves.
This results in something equidistant from the inside and outside border
I tried all scripts. What if I use a shape that is only once curve, for example a rectangle? Am I right it would only work with the script shared by @laurent_delrieu ? Is the “RemoveDeadEnds” component your own creation? Is it possible to get it as a plugin somewhere?
Medial Axis must work for single curve. But the skeleton will not be a closed curve but an open curve.
It is a line for an ellipse, a point for a circle, a cross for a square, a sort of dog bone for a rectangle. So remove dead end will not work.