The spline collision has nothing to do with how to manipulate collision at runtime for a custom terrain, or how to edit a terrain shape at runtime.
You can get the spline to snap to the terrain anytime with a simple downward line trace that starts from way up above the terrain on the visibility channel.
as any other linetrace, it can detect anything, including other actors (not the landscape) so that you can implement some kind of snapping.
What mesh the spline uses is unconsequrntial.