Secrecy is, of course, paramount, but he could probably treat his stealth fiancée with something on this side of contempt and not make anyone suspicious. A hush-hush engagement must be very little fun when one's husband-to-be prances about the place leading people on and treating oneself quite ill indeed.
Mr. Churchill and (perhaps still more markedly) Mr. Elton certainly exist to throw Mr. Knightley's virtues into flattering relief, but this ought to have been done without consigning poor Miss Fairfax to an awful marriage with a selfish louse. She, at least, has no flaws of character except an excess of reserve, which hardly counts. Not for her the thoughtless blast radius of Emma's sheltered meddling--no, all she does is treat her aunt with infinite forbearance. No one has to take her in hand to make her a worthwhile human being, and she's not even a dip like Fanny Price.
To force her to become Mrs. Frank Churchill seems unfair.