algorithmic modeling for Rhino
Looks like your curves are not consistent in directions. Try "Clipper" plug-in.
The "Polyline offset" component from clipper plug-in allows you to select the polyline as the inner offset result("H" output)measuring the length of shorter one by offsetting a polyline on both sides regardless of direction.
BTW: the classic way to check orientations is to test the DotProduct between the Z derived from a polyline Plane and some suitable "standard" vector (it could be the Z of the first plane in the polyline List, for instance). If is negative you reverse the polyline. There's another Method in RhinoCommon as well for that but requires code (and planar curves).
How it looks in code? :) For checking directionality of polyline winding?
this is way more complicated than it needs to be in GH. Just take all your closed curves and flip them with a circle (that only has to be roughly parallel to your curves) as the guide curve. Voila, all your curves are now winding consistently (and will therefore offset consistently)
Quite efficient and easy to do the double-offset with selection of shorter curve...