First of all this is great! Thank you very much for the effort. 
But the main idea is to flatten this way a selected bunch of vertexes inside a huge spline.
I mean if you have a set of 100 vertexes that needs to be flatten in one way, then another set of other 120 vertexes inside the same spline that also have their own need of flattening.
For example, you select this way (each number represents a vertex):
1,2,3,4,5,6,7,8,9,10,11,12
So vertexes 1,2,3,10,11,12 remain untouched, vertexes 4 and 9 determines the min/max Z but remains in place, and only vertexes 5,6,7,8,9 are flattened.