I seem to remember that when the announcement was made about the Sussexes and the award, someone (I can't remember the poster, I apologize), wrote that when the Sussexes learned that they were receiving this award, they weren't even going to attend the gala, but were told they had to attend in order to receive their award. I'm sure I read that, I hope I was dreaming.
If that truly was the case, then it would appear that Kerry Kennedy may have have sought the Harkles out, dangled the carrot at them, but told them they have to appear. Why? The Foundation can then run the scheme to draw donors/attendees to give big money donations to see/meet Royals. It also helped that sister Rory was working with the Harkles.