My eight year old has serious sinus issues. Her adenoids were taken out and helped a bit but it didn't solve the entire issue. I was certain she had some sort of allergy that was causing all the mucus but after tests were done she isnt allergic to a thing. The doc believes she is one of those few who just has chronic siusitus. I know what you mean about the bad breath thing though. She always smells sick to me. Some days are worse than others and when she has a lot of mucus she always gets an infection. She is old enough that we have her rinse her sinuses out daily with saline. That has helped more than anything else and it was easy.
Best of luck to you.