The author is out of his/her mind.
The clavicle bones need to rotate along with the rest of the chain setup to allow for a correct animation.
Export the animation to blender, along with a sample mesh.
Rig it. If using the mannequin skeleton just one click on my addon for blender (Bone Breaker: Unreal to Blender)
Bake positions from the ue4 animation to the rig.
Then delete and adjust frames manually.
The extra shoulder control from the rig should help to fix the anatomy as well. Its a bit of a process to clean it up. Obviously.
And/or
Just use a different starting animation pack.
I think the free set from mocap online has a peoperly made/rigged overhead swing. For instance.
Since you have 2 bones that are completely bad, and another 2 (the shoulder rotations) that are also in need of adjustments regardless, starting somwhwre else could mean less work.
Ps:
Report the animation pack to marketplace support. Link them to this topic.