I wouldn’t be unduly bothered by eating and drinking (but I would probably assume that the interviewer might mention it and apologize in advance). She might have been stepping in to cover someone else’s interview slot, or just working so many hours during the interview season that she doesn’t have time to eat lunch. Eating, drinking, and also on the phone though seems quite rude.
As other posters mention though, keep an open mind, and if you receive an offer and the school is on your shortlist, go to the admitted student day and see if it’s a ‘fit’.
My kid received a few offers on M10 and we were actively looking for reasons to cross schools off the list. One interviewer at a HADES school had been quite aggressive in their interview questioning (it was during covid and only offered virtual interviews, husband listened outside kid’s door and reported back to me that it felt more like a job interview post-college than interviewing a 14-year-old). Kid thought that they weren’t going to get an offer so had given that school barely a passing thought in the few months from the interview to the offer. I tried, in vain, to get the school back on kid’s shortlist for consideration after M10.