As follow up, I thought for havdalah drink and the drink for Saturday kiddush, a chamar Medina (such as etrog liqueur) is permitted (not for Friday evening shabbat). Secondly, is the principle of using a mitzvah object for another mitzvah an optional mitzvah, a chiyuv, or merely a minhag?

The Talmud/gemara in I believe Shabbat 117b states you should use something for a mitzvah for another mitzvah. Would using leftover etrogim from Sukkot to make wine for kiddush and havdalah fall under this category?