Reiki I is Shoden, Reiki II is Okuden, and Shinpiden is Reiki III. Levels I and II are where you learn almost all of the practices and techniques of the system of Reiki; Shinpiden is where you learn to teach. In Japan, members might spend ten years at the first two levels before anyone considered making them a Shinpiden teacher.

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.

I would never say an attunement or Reiju never worked; energetically we are all connected. What can happen is that a student is closed to the experience, or to the teacher, or the teacher was not in a whole space when offering it. If that was your experience, try another teacher. You can have as many teachers, and as many Reiju, as you like.

We are Reiki, so that is happening whether we are awake or asleep. What you can do is set your intention before sleep: place your hands on your body, bring your awareness to being Reiki, and let that be the last thought of the day. Your hands will fall away once you sleep, and that is fine.