The diagram below represents the animation pipeline.

Mocap data is imported in a format compatible with a HumanIK rig. 

This data is retargeted to the Metahuman Rig after being filtered with Butterworth to get rid of most of the noise. 

 After strugling with Human IK and the Metahumna rig though all of the initial animations I  set up a system where both HIK and Advanced Skeleton can be used.

Advanced Skeleton works much better with animation layers.  This does add a layer of complexity that I do not think Paul will want deal with. 

Refining animations involves changing the Character/Source relationships in the HIK setting, (AS/HIK in the scenes oin google drive) baking to the appropriate control rig and refining further.  rinse and repeat. 

I am thinking of baking all of the mocap data to the Advanced skeleton rigs and removing the HIK entirely. This would simplify things grealty but remove some of the data redundancy. 

This is represented by the second diagram below. 

p.s If you find the rigs slow to respond set a key on any IK handle and perormance should improve.