Amoako Boafo is in a buoyant mood. The 40-year-old Ghanian painter is about to open his first London show, “I Do Not Come to You by Chance,” at a UK outpost of the American mega-gallery, Gagosian.

When Georgetown University fellow Badar Khan Suri was arrested by federal officers outside his home in Arlington, Virginia, so began a multistate, week-long journey in custody that would end in a ...