Short time only. Due to how abrasive it is, I would only use it for a short time or you will wear through the enamel that is the tooth's greatest protection against decay.
Yes. Make a slurry with water and use as toothpaste.
Yes. Baking soda has been used successfully in many areas, including the United States as a primary agent for brushing. There are a number of companies that incorporate baking soda as an ingredient in their toothpaste formulas.