I have gone through your query regarding white spots
at the back of throat and tonsils. Though you are also having post nasal drip
and GERD. thete is also past history of smoking for last 15 years.
Considering all about your symptoms and observation of your doctor along with co-relation of described facts with investigations I would like to say that post nasal drip and chronic GERD
are also irritating the pharynx for developing leukoplakia
I would like to suggest you to consult your ENT doctor for thorough examination and necessary investigations for correct diagnosis and treatment.