Sorry, no worries then. You just did not affirmed nor declined it in the last post, so I had to ask again.
Okay, then I guess when you put the models together in Blender you first imported the original unit and then imported the other stuff you wanted to add to it, right? At least this can lead to the problem you have now. I don't really know why this problem occures, but it seems like somehow the bone rigging gets messed up somehow. I guess it has to do with some attributes we normally don't bother about. So, I made as a rule for me to always import the orignal unit/leaderhead last.
What you can do now is to import your unit again with Blender, then import the original unit, arrange your unit so that it is in the same place as the original unit, merge both meshes and then remove all vertices of the original unit. For the last part, best select all vertices in the original unit's mesh and deselect all in your unit's mesh before merging. Then you can just delete all selected vertices after merging.
After exporting the unit and nifswapping the unit should work properly.