Wow, you guys only had 1 group that did all three events (written, tour, and interview) together?
For March 11th interview, we had a full house in the rotunda and my guess would be 40 to 50 people. We were divided into three groups to participate in all three events.
Here is my guess as to why it's taking so long for the March 4th people to hear back. Since there are limited seats available before placing everyone on hold/reject, the last few seats will be much more selective. Hence, the review committee may be considering applicants from both March 4th and 11th together, as those were the last interview days for external students (apparently, I was informed by an internal student that there are still interviews for them). It will be a tough decision for the admissions committee to make.
Good luck though! 🙂