The NIF file sure can't hurt, either as a file for download or a picture of the NIF tree at least,
but helmets are anyways rather different. Yes, some are standing alone with only the head node in the NIF and rotated to strange angles, so it rotates back in-game to the right orientation, but not unlikely these also have "un-applied" offsets and/or rotations in the NIF.
Maybe check other existing helmet NIFs which were made that way to see any offsets (=location) and rotations (=angles) of the NiTriStrips/Shape node.
Then I've also seen helmets made so there's a complete skeleton armature inside the NIF and the helmet is sitting upright at the correct location on the head. There then are no un-applied offsets on the node, if I remember right, or only if the helmets are also morphed.
In any case it's always the best idea to look at existing working helmet NIFs to see how it's correctly done. Especially said un-applied offsets and rotations are easily overlooked from only a short glance at the tree.
Oh, and, for some explanation:
- "Applied" transforms are when you edit the transforms first, then hit "apply", they will zero out again in the node but directly modify the vertices themselves, so then the location of the helmet will be 0,0,0 again, but the vertical position of every vertex inside will have the offset of the helmet from the origin/ground added onto. These then can no longer be morph-controlled, as the morphs are usually made with only the helmet in mind and based on the head node at the center/origin!
- "Unapplied" transforms now are when you edit the transforms as the last time, but then "don't" hit "apply". They will be directly affecting the helmet node itself, while the vertices inside it remain unaltered. These can still be morph-controlled, as they are zero-base oriented around the head node as their origin still, as most head morphs require it to be the case.
But that's very technical now and not really needed to know.