How to split a mesh with a projected line and then shrink it?

I created a mesh from grasshopper, converted to subD, and projected lines on it.

Top view

And Now I want these projected lines to be the edges of the subD surface, so that I can shrink with the surface with those lines. Would it be better to work with Polysurface instead of SubD in this case?

Any tips would be greatly appreciated!

And is there a way I can reduce/undivide/merge SubD and Polysurface faces? Like the feature that does opposite of “Subdivide SubD”.


Hello- no… what might work, though only partly, it may not match the outline exacly, is SplitMeshWithCurve until you have that triangle and then ToSubD.