No in fact I think it is very helpful. Just because they are your advocate doesn't mean that their opinion can't change, nor does it mean that other member's opinion doesn't matter.
No, there is a tedious discussion involved, but the purpose of interviews if to obtain a first hand opinion of you. That obviously is going to be heavily weighed in the decision, and thus can play a significant role. As far as I know it is just one large committee that goes student by student in the very end, but honestly I stay out of the admissions decisions and thus don't have any first hand experience.