Hi 'shoutlouder', go back to your ENT doctor and request for an allergy test! What does he mean by saying that he
thinks it's allergies. It can only be confirm by an allergy test. It may or may not be allergy that's causing your problems since you didnt mention itchy nose or sneezing which are 2 'main' symptoms for allergic rhinitis sufferers.
The yellow mucus could be a sign of infection and might mean that you have sinus infection. That can be checked out by doing a CT scan to see if your sinus are indeed infected.
The Nasonex is a nasal steroid spray and is good if used long term for problems like congestion/stuffiness, runny nose... If you have allergies, the
clarinex(antihistamine) will help stop the allergic reaction in your nose. The coughing that you have could be due to the post nasal drip(PND) where the mucus flows back into your throat and cause irritation and make you cough. Hope your doctor can tell you what your problem is!