IK and FK complement one another. An example is the arms movement in a sword thrust. The thrusting arm is controlled and determined by the movement of the sword (IK; control) whereas the opposing arms swings backwards (FK; expressions) to provide a balance force.
Mocap data is baked FK data, even when the movement it describes would be an IK. You will need to move between IK and FK systems in any animation. Sometimes, you gotta fake it.
A way to fake it is to copy the control curves from one system and paste them into the other system. An example would be controlling a ball in the hand of a character, but retaining the ability for the hand to do other things.
FK skeletons are not all the same. Where you start the chain matters. Top rooted FK’s are bad, for example, at sit-down movement. You may want to change a frame of reference of a control instead of its chain.
Each bone in the hierarchy has a sibling that receives the same rotations, but it is doing nothing but twisting. Then you skin to that sibling.
Spline IK is good for only one thing, the spline of motion. The bad thing about the default one is you are moving a lot of individual control vertices to do one thing. You are setting a lot of keys for one thing, that individually have no meaning.
