Spline IK is used for more advanced spine, snakes, tentacles and rubber man.
When you set the spline IK, do not check the create curve option.
Use the CV curve option 1.
After proper joint orientation setup, make controls then duplicate and snap to each joints. After that, move custom controls strait backward.
After you make spline IK, convert every CVs into clusters.
Click a control and a cluster then parent constraint.
Do for everything.
Make the hierarchy for controls and group it then move the pivot point on the root(pelvis) joint.
Use the connection editor, make a connection between chest control(top control) rotate Y and twist attribute in IK spline.
This is a final scene and shows what I have done for cleaning up.