Trying to get some basics straight - icon is self explanatory, what shows up in the inventory. But helmet.nif is what appears when you wear it, helmet_gnd.nif is what you see when you find it laying on the ground? Any reason those couldn't be the same for stuff like helmets or weapons that don't change shape?

Also I'm having no luck extracting to copy built in stuff from BSA files, NifScope supposedly can do that but I'm just not getting it. Is there a simple step-by-step guide with examples someplace?

You would want some interior details in case the helmet rolls around and you see the inside. Not sure how much you would need there. Also, the helmet could change shape slightly to be placed on the head without clipping, but need a different shape to look right on the ground.

How are stock helmets handled? Do they have a nif for the character and one for the ground?