yes in the ai character you would create a variable and its type would be pointofinterest as in the point of interest blueprint that you were trying to cast to before. if you wanted to change your system and use target points or something else then that would be fine as well.
yes you cast to the class of the aicharacter, so if your ai character was of the class “BP_MeleeEnemy” then you would cast to that class. the class needs to be as specific as the variable that your looking to get.
A little basics on casting say you wanted to see if a object was a character then you could just cast to character, but if you wanted to see is it was a specific character with certain attributes such as checking if it was a boss then you would need more defined criteria to compare to so you would cast to bosscharacter class which would be a class that all bosses inherit from.