Newaris are not directly descendents of Tibetans. People like Sherpas and Bhotiyas are. Newars have far more complex history, since they have lived in Kathmandu(a crossroad) for thousands of years.
yes, where exactly did they arrive from? granted this is not something that is understood or studied in depth, but i think it is a safe bet they did not come from India. Bhote literally means tibetan that would be refugees.