By God, you have solved it for me! They were sitting among their respective groups and inside these groups they were seated alphabetically according to their countries:
- reigning European monarchs: Denmark, Luxembourg, Norway
- other royals from reigning European monarchies: Belgium, the Netherlands, Spain, Sweden
- former European monarchies: Bulgaria, Greece, Romania, Yugoslavia
- Monaco is still strangely placed here
- reigning non-European monarchs: Brunei, Malaysia, Swaziland, Tonga
- other royals from reigning non-European monarchies: Kuwait, Lesotho, Morocco, Oman, Qatar, Saudi Arabia, Thailand, United Arab Emirates
Thank you very much! Years of speculation are finally over