Is it true that once an addict always an addict?

Yes? From a biological standpoint yes in my opinion, people can be sober 10, 20 or more years and still relapse severely if the substance is reintroduced. From a recovery standpoint identifying as an addict is very helpful for most. It keeps people in recovery; grounded, humble and reminds them where they were. Some don't take this same view which may work for them, but most can't escape their biology.
Nothing is absolute. While there is always a great risk of losing control with any substance, i know several people with history of addiction who went to controlled use especially if it was a substance of a different class. Years later, i wait still for the relapse. However, I have seem others fall apart with minimal use even after decades clean. I suspect the second group is the majority. Don't take a chance.
See below. Depends on whether the behaviors indicate a disorder. In psychiatry, drug "dependence" is the term for addiction. Any drug dependence carries a long-term risk of relapse, but treatment can reduce this risk and improve your quality of life.