I'm not sure what you're asking in the first question, but I'll take a stab at it.
Arthritis is definitely diagnostically significant in diagnosing lupus.
Arthritis, as you know, is inflammation of the joints. It can manifest with pain, swelling, heat, redness, limitation of movement. It is not to be confused with arthralgia, which is merely joint pain. Whether or not your headaches are from vasculitis, would be hard for me to say, but it's certainly a strong possibility.