i don't know the answer to how many mutations are required for NS5A resistance, but that data may be out there if you google early abstracts on the drug (particularly on monotherapy or in vitro)
i think 3DAAs are 1.5-2 years behind 2DAAs, so i was really implying they will enter the clinic in a year i'm not sure BMY will necessarily have as big a lead as you think if companies start increasing collaborations like the one BMY and VRUS are doing, but time will tell