] Unlike Catherine, Henry Tilney speaks well enough to be unintelligible.

Very good, Kay! ;-)

Yes, I agree - Catherine would infer the most kind, polite interpretation to anything anyone said - look at how she puts a positive spin on practically everything Isabella says and does! But Henry, again, is being very clever and acerbic and knows his audience and knows he can get away with dissing Mrs. Allen without Catherine being the wiser.