There are a few shoulder pad thingies listed as hats, and I think those should belong with accessories givin that they do not cover the head or face, only the shoulders. Anything else that does not cover the entirety of a head or face should be an accessory. I think those 2017 glasses, as an example, could be an accessory.
Layering and stuff won't be an issue with full helmets and accessories mixing because people will either go for a good look or a goofy look, and people do that in MMO's.