No. The system of Reiki was founded in the early 1900s by a man who worked hard at his own healing and spirituality and passed on practices for doing the same. It is a subjective practice of self-development, so nobody can scam you with it; the only scam is someone charging a great deal of money and giving you nothing. As in any field, there are dodgy people. Check your teacher, and keep to the good ones.

You cannot stop being Reiki, but you can lose strength and sensitivity. Practising the system of Reiki cultivates your own energy: meditation, hands-on healing and chanting feed and nurture the ki at the hara. Stop, and you lose a little awareness. Keep going, and you become the precepts and your life gets healthier and happier.

No. In the Japanese system the attunement is Reiju, a spiritual blessing, and you cannot have too many blessings. Receive as many Reiju as you can, let go of the mental fuss, and simply sit in that healing space remembering who you are.