I would recommend having a separate skin for each item (a skin for each helmet, each pair of shoulder-plates, each pair or trousers, and so on) and then combine the equipped skins to a new skin. The following posting by Nate lists a few URLs which might be very helpful, if you haven't already checked it out: Combining Skins
Regarding feathered helmets and helmets without feathers: When using skins for each item, you could use Skin Bones to have some bones only active when the feathered helmets is equipped. You might also want to have a look at the Spine example project Mix-and-match
which makes use of Skin bones. Every bone with a skin icon (the orange T-shirt) next to it is a skin bone, only enabled when the respective skins are active.
In any case, you should not need to attach a blank png image at all, it should be easier to just not add this attachment at a skin (or delete it in case it was added before).