Answers to your questions about FEV1/FVC ratio:
Is the low-normal ratio a cause of concern? Absolutely, not a cause for concern.
Does this ratio remain the same over time or does it decrease? The ratio slowly decreases over time and the table of normal predicted values takes this into account.
Could ratio been impacted by seasonal mild sniffles? Yes, especially since your doctor described your airways as “hypersensitive.” That hypersensitivity, is usually described as increased bronchial hyperreactivity.
Is there anything I could do to improve it (I don't smoke and try to avoid all exposure) No, there isn’t anything you can do, unless the borderline ratio is on the basis of asthma, in which case an inhaled long-acting bronchodilator might improve your FEV1.
With mild COPD, how does it progress for non-smokers? Is it more-than-likely to lead to a moderate level
and how quickly? The problem is usually expressed as yearly rate-of-loss of FEV1. The rate for non-smokers is, on average, 30cc/year. For active smokers the rate may be 100cc/year or more. When one stops smoking, the rate of loss progressively diminishes until, after 5 years, the rate of loss approximates 30cc/year (the rate for non-smokers).
The truth is that you do not have to worry, having quit smoking. Please stop worrying. Your lung function is fine and should remain fine, the rest of your life.