There are elements of QA, and ongoing training, but obviously, there are only two people in the interview, so all that can be reviewed are artifacts of the interview. It is an interesting question. I would say that if an ordinarily good, competent, interviewer delivers one “strange” interview, then I doubt that I would catch that. However, if I have a regularly poor interviewer, then I am confident that that would be caught, but usually of course only after a few bad interviews.
I cannot speak for other universities. As far as I know, MIT is the only university which does grade its alumni interviewers and when I talk to regional chairs at other reasonably peer schools, they are quite taken aback by that. I can confirm that the overwhelming majority of “strange” interviews that I look at did not arise from voluntary feedback, because, as you say, people are loathe to come forward. I would also add that just as MIT is familiar with most of the secondary schools that regularly send applicants, they too are familiar with the ECs and know how to interpret our reports.