For your first question, understand that a swipe gesture is not a momentary event. While the swipe is occurring the Leap Motion software dispatches a SwipeGesture object in each frame, updating the current position of the swipe. If you are switching your phone view every time there is a swipe to the right in a frame, you could be switching phones over 100 times a second.
Another possible pitfall is that swipes are associated with fingers, not hands. Thus a single motion of the hand could create 5 swipe gestures. If your code reacts to each one, you will get more switching then you bargained for.
Other than that, there is no inherent sensitivity of a swipe gesture -- that is entirely how your application reacts to the swipe data.
On your second question, the Leap Motion range is not adjustable. The range from foot to waist is a bit to large for average height adults.