Date: 13 Shevat 5591
From: Rabbi Moshe Sofer
To: Rabbi Moshe
Two questions are addressed: whether ten who prayed individually may reconvene for Kaddish and Kedushah, and why intentional omission of Kiddush can be remedied but not Havdalah. The Chasam Sofer resolves the first by distinguishing Sephardi and Ashkenazi practice regarding individual Kedushah, and the second by explaining that Kiddush relates to progressive day-sanctity while Havdalah marks only the transitional moment.
Source: Shu"t Chasam Sofer OC §17