Yes, only sterile, disposable needles are used. Every licensed acupuncturist has passed the National Clean Needle Techniques Examinations.
Discomfort is mild or nonexistent. Most people only feel the initial pinch.
If acupuncture is provided by a well-trained acupuncturist, the most common side effect is an occasional bruise.
Twice a week for 5 weeks is better than once a week for 10 weeks for acute conditions. For chronic conditions, twice a week for 5 weeks, then once a week for another 10 weeks is recommended.