Is it permissible for a fiancé to agree with his fiancée's father for his daughter to attend celebrations and events that he deems to contain Sharia violations, with the intention of preventing her from doing so after marriage?

The condition set by your fiancée's father, that his daughter attend singing parties and forbidden mixed gatherings, is invalid and must not be agreed to. Rather, he should be advised and reminded of Allah. If the girl agrees, then outwardly agree to complete the marriage contract and rescue her from her corrupt environment, choosing the lesser of two evils.