Your symptoms suggest moderate to severe persistent rhinitis as CT paranasal sinuses
have excluded anatomic abnormality or opacified sinuses (i.e., sinusitis
Allergy testing is an important part in the investigations for persistent rhinitis/rhinosinusitis.
If this is a problem throughout the year (i.e., perennial allergy
type symptoms with sneezing, runny nose) then house dust mite allergy
is highly likely. Occasionally animal dander allergy (cat, dog) is also a cause but you should be able to determine that yourself quite easily.
Skin testing to exclude house dust mite, animal dander, tree and grass pollens must be done in your case. Blood tests (specific IgE testing) is available for most allergens although it depends on the local healthcare provider. The dry mouth
and throat, 'drainage' that you complain is called post-nasal drip and apart from allergies, gastro-esophageal reflux disease (i.e., excess acid secreation from stomach) are the usual causes.
Treatment can only be recommended once the specific cause is identified or a trial of treatment may be initiated if testing is not possible.
Hope that was useful.