On the plane, the billionaire noticed his former lover seated just rows away, accompanied by twin boys who looked clearly like him.

Ethan Cross, a billionaire who built his fortune from scratch, rarely boarded commercial flights.   But today, due to an unexpected issue grounding his private jet, he reluctantly agreed to fly first class on a public airline to avoid missing his keynote at a global tech conference in Zurich.     The lavish cabin—its champagne,…