I used be a former alumni interviewer for Duke, and I know the process well. The Duke interviewer does not receive your grades or scores. In fact the only thing they will get is your name, high school, email address and your potential major. And whether you apply to Trinity or Pratt. Thats it.
The alumni interview is almost meaningless. Whatever they write has very little influence. So as long as you don’t completely botch the interview, it won’t really have an impact.
Having said that, your chances are tied to a lot of other factors, namely institutional needs: geography, URM status, major, full pay status, etc. There is a good article in the Duke Chronicle that outlines how the Dean decides who to admit in a process he calls sculpting. Good luck!