2. Yes, probably, though the amount of flickering is not so much the point,. What's important is the meaning of the flicker. If the duration were important, the writer would probably have said "the taillights were flickering"
3. The meaning of the flicker is that the person was driving away, or preparing to drive away, and then tapped the brakes because he or she considered stopping (i.e changing his/her mind about driving away). Why the person was driving away is probably significant, and the 'flicker' shows he or she is uncertain about the decision for some reason.