I can't open your part, so this is going to be a little difficult to work out. If you could post some images it might help.
A two-point spline without any tangency conditions should be a straight line. If it is not a straight line, then it must have some other relation that is forcing it out of a straight line. You could have added these relations by manipulating the handles or the control polygon.
If a spline is tangent on both ends to fully defined arcs and is set to "proportional" in the properties, then it goes black for me. But I can't move it around on the sketch, as the arcs lock it in place.
If a stand-alone spline is set to proportional, then it can be moved around on the sketch. If one of the end points is fixed, then it will rotate or scale as you drag the other end point.