We can guess that it may at least be MMS117 arms as opposed to a total reuse of that entire body. The neck looks suspiciously DX10ish in its proportions and if this is indeed a reworking of the DX10 head then that's another reason to think that the core body is of the DX10 ilk.
We haven't seen any poses with the arms with more than a 90 degree bend of the elbow which could either be because they are MMS117 arms or because these people can't pose for ***t and aren't aware of the full range of motion the arms are capable of. No shot I've seen, of either the initial proto shots or these toy show pics gives any indication if these arms are doublejointed.
That´s prolly the point, cause for proportions and articulation it would be totally stupid to go back to the MMS body, or not?
But your other point seems valid too.
We´ll have to see...