I have the same terrible habit and everyone that I know tells me to stay away from word fillers, better yet pause instead. It shows you're thoughtful. I think it depends on how many times you've said umm, but you have to remember that they know you're nervous so don't completely disregard that school. You may have impressed them and just not know. I'm also apply in this year so this is definitely subjective.
Yeah you're overthinking it when you start evaluating your eye contact and posture a week after the interview ended. You probably did better than you think and you'll have more interviews to worry about, so forget about this one!
Don't worry about it.... in one of my interviews, when asked what my biggest weakness was I told them I was rude. Oh, and I was late (my hotel forget they were shuttling me, but nonetheless that doesn't reflect positively on me). However, I got a Dec 1 acceptance from them...
This may sound counterintuitive, but at all the schools from which I received Dec. acceptances, I interviewed less than optimally. That is, I stuttered quite a bit and more than likely came across as anxious and awkward. If your stats are above average and you were genuine with your answers, then you are probably fine.
Multiple um's won't break your application culminated from years of (presumably) hard work and dedication. Sit back. Relax. And try to enjoy the process.
I think for a good amount of situations, people tend to negatively reflect their performance (exams, interviews, etc), without even acknowledging the amount of hard work, drive, and passion they applied for that specific scenario (and not even thinking of the positives).....I do this too!
From what it sounds like, you will be fine! Everyone is a bit nervous, especially with interviews. I'm sure you said a lot of reflective and honest responses that interviewers liked to hear; a little bit of nervousness won't kill your chances!
Just relax, continue to do well on the other interviews, and you'll be surprised that when Dec 1 comes, that so called "non-optimal interview) will be a school that accepts you
Keep up the good work, and best of luck with the rest of the cycle!