Alright Union I went through your file, so let's start from scratch here:
1. Draw a linear spline parallel to the floor. Just 2 points, start and end. Make your spline Bezier. Name this "Control Spline"
2. Duplicate that spline, choose the 2 vertices, right click subdivide.Choose again the new vertices and subdivide again and then one more time.
3. Choose all the vertices, go to Character/Convert/Spline to joints.
4. Name the root of this Chain Spline IK.
5. Take the first joint and apply a Spline IK. Use the end joint as the end and the CONTROL SPLINE with the 2 points, as your reference spline.
6. Now go inside the Spline IK tag and press Add & Create twice. No you have 2 handles that will control the curve of that spine. Essentially what they're controlling is the tangents of the bezier curve,
7. Zero out/Freeze the two nulls you created. Inside the tag of each null color them and shape them to taste.
8. Use protection tag on Translation/Scale and all rotation axis apart from P.
9. Now try out that spline curving, and go back inside the IK Spline menu to play with the distances. See my previous picture to pick on the ratio: Head to Tail controlling distance is almost 1/5.
10. Now it's time to create the head and tail joints. I think you know how to do that. Just parent them accordingly (use the image above)
11. Zero out everything
12. Don't bind your mesh until your joints are working as they should .