Is he required to offer expiation, and is he sinful, for caressing his wife (with kissing, hugging, and superficial actions) before the agreed-upon wedding, knowing that he did not consummate the marriage with her?

You are not sinful for performing these actions or for breaking your promise to your wife. She is your wife by contract, and what is permissible for a husband with his wife is permissible for you with her. Fulfilling a promise is desirable, not obligatory, according to the preponderant opinion. It would have been better to fulfill the promise, especially if custom prohibits such an action. You should hasten to complete the marriage, as Allah Almighty says: "So hasten to [all that is] good."