Historically, Worawongsathirat and Sri Sudachan were killed in an ambush set up by Lord Piren-Thorathep when the couple were going on a trip to catch a supposed white elephant.
The witch is killed by the queen's black body guard brandishing a huge sword. There were no black people - slave, body guard or otherwise - at the court of Ayutthaya.
The "clandestine" ninjas generate such tremendous noise as they trespass on the palace in order to assassinate the king that it's surprising they don't wake up the entire town with their ruckus, let alone arouse the attention of the palace guards directly above whose heads they trample across the roof beams like a stampeding elephant herd. The audio FX in the entire scene is so grossly exaggerated that it's almost comical.