My reply's probably a bit outdated but for the sake of future generations reading this...
There's no difference between student interviewer or adcom. If you interview on campus, it will most likely be a student interviewer, unless there's not enough students volunteering that day, then adcom/other staff would fill in.
Also I would say, it's not unusual for SOM interviewers to send thank you notes to the interviewee first

It's just to show our appreciation that you took the time to come down all the way to New Haven to talk to us!