Ah. I obviously failed to make the Theon/Sansa/Myranda--Vader/Luke/Palpatine connection. Well spotted!
I did notice, however, that Myrcella's death scene--due to poison, while on a boat--apparently referenced Richard Wagner's tragic opera Tristan und Isolde (where in Act 1 the heroine tries to poison herself during a sea voyage).
Myrcella's fiancé was even named Trystane, making the connection more obvious.