Yours is a very common history of asthma that varies in intensity over time through a lifetime, often for reasons that are not immediately obvious. The great response to Symbicort is very encouraging and the best indicator that you have controllable asthma. I would interpret your “normal” pulmonary function tests in that same manner.
I can understand why your having “normal predicted” PFT’s, at any given time when the asthma was not overtly active but that normality would not rule out the diagnosis of asthma. One’s PFT’s can be in the normal range when the airways are still inflamed but not constricting. Any measurement of pulmonary function, in an attempt to “pin-down” the diagnosis should be performed at a time when you are symptomatic. The Methacholine challenge is generally quite reliable test, but there are instances, albeit rare, when the test is falsely negative for a person who otherwise undeniably has asthma.
It is wise for your lung specialist to consider other lung diseases, or diseases of the vocal cords, that can mimic asthma but ultimately the ability to effectively control your asthma symptoms with standard asthma drugs will be the most important matter for you and your doctor.
Also to add to my previous comment. I get symptoms from dust, smoke, chemicals, perfumes, cold air, and maybe wheat/gluten. Thats when the coughing or shortness of breath starts. Also after exercising I cant stop coughing. When I was on Symbicort I didn't have any symptoms.