The Epic Mannequin is not sutied for work on animation retargeting between any character and the Epic skeleton. The mannequin mesh is completely atypical with non continuous joint meshes, instead having some sort of mechanical joints and then some non deforming shoulder plates on top of it. It is impossible to see wrong mesh deformation while doing animation retargeting. Also the mannequin is completely useless as an overlay when trying to get a retarget pose on another skeleton because the arms are not linearly aligned with the bones.
The solution is to make a simple mannequin that is rigged to the epic skeleton and that consists of nothing but simple shapes like cylinders and which is basically just a thin skin over the bones. This way it conforms with the bone structure and can be used as overlay mesh in another skeleton when trying to get the bones aligned to the Epic skeleton in the reference pose.