Re: Of cabbages and kings....

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The engagement had to be kept a secret (in their minds at least).

It had to be kept a secret because Frank's aunt and uncle (the Churchills) could have withheld his inheritance if he married against their wishes. It was not an unrealistic scenario, not something just in their minds. The correct behavior, of course, would be not to enter into a commitment until they have no obstacle to the marriage (according to the era's requirements).
As for Jane Fairfax, I would be hard pressed to become friends/be open/intimate with a girl my boyfriend continuously flirts with (and who flirts back, of course). Frank Churchill behaved abominably, both to Jane and to Emma.