If I began my Purim <em>seudah</em> during the day, but it did not finish until after <em>tzeis ha’kochavim</em>, do I still recite <em>Al Hanisim</em> in bentching?

Yes. However, there is an opinion that if one already davened Ma’ariv, then they should no longer recite Al Hanisim in bentching. Therefore, to avoid this question, it is proper to bentch before davening Ma’ariv. (Mishnah Berurah O.C. 695:16)