An answer to your second question - early ads claimed that smoking soothed sore throats. When mentholated ciggies came out, they made that claim again.
As to you first question, you must be aware that on some levels no one can answer that question. How could anyone know? What effects could smoking have had on your body that will not be reversed? There’s no way to know. But, the (idiotic) implication of your question is this: how long can I smoke and know for sure that it hasn’t or will not hurt me? I mean, does this mean that if someone tells you the answer to your question is 6 years, then you’ll smoke just that long and then quit? Or are you asking how much of what you hear or read about the dangers of smoking are absolute truth? What’s true? Remembering that cause is very difficult to prove, just keep this in mind: In epidemiological studies here is a strong positive correlation between smoking and numerous health problems. That means that the occurence of these health problems is much higher among folks who smoke, and the more they smoke, the more likely they are to contract such problems. What are you really hoping for - someone to tell you that it’s all a hoax? You’re free to continue to smoke and it’s a perfectly harmless habit? Not bloody likely, young feller.