The answer is an unfortunate yes, you can cheat at online chess. While many of the online chess sites do have anti-cheat measures in place, it’s impossible for them to police and prevent all forms of cheating.
Yes, online chess can be addictive though the extent to which internet chess addiction is found in the population is not known. Online chess addiction is likely to be fairly rare and if it is a problem for someone they ought to be able to break the habit with the aid of a mental health professional.
Yes, you can get a FIDE online rating, which is a rating awarded specifically for online play in the FIDE online arena. However, if you want a full FIDE rating then there is only one way to obtain it and that’s to play in a sufficient number of competitions, in person, and win a sufficient number of games under FIDE’s watchful eye.