Natalie,  I can tell you from experience, you do not have to appear to have a cold at all to have sinusitis.  I suffered with chronic sinus problems for years and did not generally appear to have a cold - only occasionally did I have the nasal congestion, runny nose, etc.  A lot of it may be related to allergies or what they termed "sensitivities" to certain substanced.  As far as how much can you daughter take without going into crisis - I guess it depends on the individual, but my daughter has had sinus infections before with slight fever and it never precipitated a crisis.  Of course, all of our children are different.