Will the husband be held accountable for his wife’s interactions with men in her mixed-gender workplace if he forbade her from speaking to them in a manner that goes beyond work-related matters, and she disobeyed his command, and if she laughed with her colleagues during work, under the pretext of politeness, knowing that he had forbidden her from doing so?

It is not permissible for a woman to join work that involves unlawful mixing. The fiancé's jealousy over his fiancée and his reminding her to adhere to Islamic regulations in dealing with non-mahram (definition) men is correct and not extremism. The fiancée is a non-mahram to the fiancé until the marriage contract is concluded, and he should treat her as such. If he marries her, he is responsible for her and must prevent her from anything that could lead to her corruption, in accordance with Allah's saying: "Men are in charge of women" (An-Nisa: 34).