Just to clear up what Lois Lane stated, you won't get much help going to an orthodontist for root canal. Look up an endodontist, they are the ones who do root canals. A general dentist can do root canals, but the more difficult cases will be referred to an endodontist, who specializes in this procedure.
Buglover, have you seen a doctor who specializes in sinus problems? One of my SIL's has a lot of sinus problems and has tried all sorts of treatments, surgery, etc. Maybe the sinus symptoms would be covered by your insurance.
Another idea, several people on here swear by the Neti Pot. Maybe something like that would help your sinuses.