I wouldn't bring up the fact that you were told to improve your MCAT but didn't. If they ask why you were waitlisted just say you weren't sure and felt that maybe it was for lack of clinical/work experience (which is something you improved on).
I would definitely go, medical schools don't like wasting their interview slots and this is your second time interviewing at the same school (which I would subtlety mention), it would be very odd to interview the same applicant twice just to waitlist/reject them.