After the camera pans the dead bodies, it cuts to a long shot of Prince Ludwig, disguised as the Queen, holding the knife pointing straight up. When the camera cuts to a close-up shot, he's holding the knife a different way and pointing it down.
"Bastard son of a bitch" is a common insult in English but its literal translation is not typically used in Spanish. The torturer should've been finished at "bastardo."
"Head over heels in love" is an English expression with no idiom equivalent in Spanish.
Percy sings the Pat a Cake nursery rhyme in this episode. The series is set during the reign of Elizabeth I, which ended in 1603. This nursery rhyme was not published until 1698.
Blackadder communicates with the Spanish torturer by playing charades, and the torturer says "sí, sí, sí!" (Spanish for "yes, yes, yes") when Blackadder guesses correctly. If he doesn't speak English, as depicted, he shouldn't react to Blackadder guessing the translation correctly, because he doesn't know the translation.
In his pantomimed conversation with the torturer in Prince Ludwig's dungeon, Blackadder interprets one of the torturer's gestures as meaning that he (Edmund) is "head over heels" in love with Satan. In fact, at that point the torturer had his heels over his head instead of the other way around.