1- They can get punished for not learning that it’s haram. If they tried to learn and missed this one, then Allah forgives them for not knowing. But if they didn’t really make the effort to learn, Allah will hold them responsible for not learning.
2- Yes you should tell them. It’s your obligation. But if they reject and are stubborn, then you are not obligated to keep telling them. As long as you made them aware that it’s haram, you fulfilled your obligation. Yes it’s good to be creative and find effective ways to advise them.
3- No. If you are sure they won’t stop and it won’t have any effect on them, you don’t have to tell them.
4- It depends. If the person generally cares about his religion and tries to learn its rulings, then he may be excused. But if he’s negligent and doesn’t really care about religion, and that’s why it never occurred to him to learn about it, then he is not excused.