I think you need to have your vet culture the nasal discharge to find out if this is a bacterial or fungal infection and which antibiotic or antifungal will work on it.
If your vet cannot help you then ask for a referral to a specialist in internal medicine or other specialist who may be able to help more.
If your pet was recently in the southwest or California before this began it could be Valley Fever which is a fungal disease but there are others too.
This site discusses nasal issues
http://www.petplace.com/article.aspx?id=321
With the coughing and nasal discharge culturing the infection is the best bet to finding what is really wrong and how best to treat it.
Hope this helps you!
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