She went missing July 2 and wasn't found until July 8. That's 5 days/6 nights for whoever did this to disappear himself/herself (doubt it)/themselves as well. When did the conference wrap up? Wonder did any attendees leave before her body was located? One MSM article I recently read stated that 80 conference attendees helped search for her initially.
Point is how to interview anyone and everyone who had or may have had contact with Suzanne. And how to nab a stranger who could have crossed to other parts of Greece on a ferry or a plane (Athens included) and be long gone.
The only reason to worry about people who left at the end of the conference is if there is some reason to believe that someone she met at the conferences in the last 5 years is the culprit. That seems unlikely.
The conference ended on Friday, July 5. She was disappeared on July 2 and found on July 8. As for interviewing, there's very likely a digital trail to the cave, one that is also at the academy. There may be ways to identify the suspect without detaining everyone who was at the conference.