Best way to "expand" or "contract" a closed spline?

I have 2 questions:

1.) I was given 3-D coordinates of a spline outputted from another software, so what I did was create a 3-d sketch, plotted these points, then created a 3-d spline through them. Next, I created a plane through 3 randomly selected points (several points between each of the 3 points). Next, I mapped the 3-d spline to this 2-d plane by creating a spline having the same number of points and creating coincident relationships between each point in my 2-d sketch to the corresponding point in the 3-d sketch.

This method seems to work well, just wondering if there is a better way to do something like this?

2.) Now, I've got my 2-d sketch of a closed spline shape. I want to be able to keep the shape of the spline constant, but change its length/perimeter, i.e. expand and contract the spline without changing the shape. So I deleted the coincident relationships to the 3-d sketch, checked "Proportional" to maintain the shape, but am having trouble expanding and contracting the spline easily. I want the center of the spline to remain put while I expand and contract for visual confirmation (with the 3-d sketch) that the shape remains constant. What is the easiest way to expand and contract the spline? I tried using tangent circles and distances to a circle but am not having luck (the spline moves instead of expanding, and fixing a point on the opposite side does not enable me to keep the center of the spline in place). In other words, I want to expand or contract the spline in all directions, not just one. Any tips?

Thanks in advance.