As I understand it, you have one spline ("Spline-1") in Sketch-1 that is fully dimensioned so that you can have 3 Configurations (A, B, C) (with different dimensions). Now you have a different spline shape ("Spline-2") that you want to use for a new configuration (D).
If you want both splines in the same sketch (Sketch-1), you can use the "Selected Contours" option to select which spline you want for this Cut-Extrude-1 feature. So no need to switch splines to or from construction geometry.
But, you cannot select Spline-1 in configurations A, B, C and Spline-2 in configuration D.
So stick with selecting Spline-1 and use Cut-Extrude-1 only for configurations A, B & C.
Thus you need to add a new Cut-Extrude-2 feature (active only in configuration D) that still uses Sketch-1 but selects Spline-2 as the contour.
Not what you were hoping for, I know . . .
Exactly as you stated, and not the news I was hoping for. However, Good to know, so I shall workaround.
Thanks for clearing it up!
Attach your *.sldprt file here and end all doubt.