I don’t own an iPhone so I baked the sample animation and retargeted the result.
However if I apply it with body animation the head will not follow.
I can copy paste the facial keyframes from the sequencer ( which takes about an hour of freezing up) to my custom ones and that will follow different body animations.
So apparently some extra transforms added baking, unticking transforms for baking crashes Unreal editor every time so cannot test that.
I want a facial animation to use with any body animation basically so my NPC metahumans are not frozen faced,
I can parent heads to bodies including headless dressed DAZ people and change the pivot but I cannot hide the part below the neck, I crash Unreal every time I try vertex paint to create an alpha as Google search results suggested, do I have to give everyone huge scarves and cloaks??? 
OK I changed the parent to the upper chest and it stays in place better at least.
Now I have a new challenge, how do I centre the pivot on a head attached to a child socket in the ThirdPersonBP, trying to move it in the viewpport of the Blueprint crashes Unreal Editor (well my GPU actually) but alt mouse wheel doesn’t seem to work in that viewport anyway and I tried creating a new skeleton and no luck there either, moving the neck and head just stretches the mesh.