Frederic is the Pirate King's second in command.
After rejoining the crew, Frederic seems to be acting as the second in command, even giving orders such as "hush hush, not a word," and hardly leaving the King's side. Since Frederic was the Pirate King's apprentice he would have been expected to learn and become proficient in his trade by the time he finished his term of apprenticeship, originally assumed to be when he turned twenty-one. That trade being captaining a pirate ship. Since Frederic was effectively being trained as the King's successor, it makes sense that he would act as the second in command.- Well it also seems like Frederick is the most competent pirate (even more than the Pirate King himself) so it makes sense that he would become the second in command.
Major-General Stanley is a Time Lord.
The proof is that his "I Am" Song is sung To the Tune of... an old Gallifreyan folk song, "I am the very model of a Gallifreyan Buccaneer."
His Time Lord status also explains:- Why he's familiar with the 1878 megahit H.M.S. Pinafore, even though Pirates (according to Frederic's math about leap years) evidently takes place several years earlier, in 1873.note
- Why his military knowledge only extends "to the beginning of the century"; he's been too busy learning knowledge from the past and the future to keep track of the present!
- Why he puts such value on ceremony (introducing himself in full before finding out what danger threatens his daughters, establishing whose ancestors legally and technically belong to whom, wanting the policeman to "receive a general's blessing" before they leave, etc., etc.) and oligarchy ("We love our House of Peers!").
Ruth is the true Pirate King.
After Frederic spurns her, Ruth returns to the pirates. Almost immediately thereafter, they discover the paradox of Frederic's contract, binding him to their will. Despite supposedly only being an aging child's nurse, she joins in the invasion of Tremorden Castle. In the finale, she's the one to reveal that the entire crew are not merely orphans, but orphaned members of the House of Lords, something she'd only know if she had reviewed the entire crew's papers.The Pirate King himself, in contrast, is easily led by his emotions, and always ends up reinforcing Ruth's plans (whether for Ruth to join Frederic, to manipulate Federic further, or to march on and destroy the Major-General's home).
The Pirates started their piracy as children's play
At the beginning of the play the pirates reveal that they are all orphans, and at the end Ruth reveals they're noblemen and they resume their titles and legislative duties. This may seem like a large contrivance, but one possible explanation is that before they were pirates they were just children whose Peer fathers went on vacation in the same place and died in some tragedy there. These children, suddenly left on their own, decided like Tom Sawyer to become pirates. That's why they dress and act like they're in the Golden Age of Piracy despite being two centuries late for it - those were the kind of pirates in the stories they read. It can also explain why they at times act childish and why Ruth was a "maid of all work" - they grew up without any adult guidance.