Matilda describes Pernilla Ogilvie as "sail[ing] across the room like a man-of-war." How does this simile reveal Mattie's attitude toward Mrs. Ogilvie?

Simile or Comparison:  it is a rhetorical figure that consists in comparing a real term with an imaginary one that resembles it in some quality.

Explanation:

The one that matilda describes pernilla as a warship walking around the room, is making her interlocutor see that pernilla is has a strong temperament and a rigid and dominant character, that he is alert, ready for war to ignite discord before any signs of alarm or suspicion.

i suggest it is anadiplosis

hm, switch the relative; nominative and indefinite; objective ? not too bright in the knowledge of the english.