In 2013, after almost a half-century in exile in London, Constantine II, the deposed king of Greece, quietly moved back to Athens to live “as a commoner”while his former kingdom teetered on the brink of economic catastrophe. Despite the awkward timing, Prince Nikolaos, his devoted second son, relished the moment. He was raised in Britain but, fiercely proud of his family and heritage, had always dreamed of living in Greece. It was a dream that his new wife, Princess Tatiana, was still getting used to…