Helping with maintenance tasks like cleaning the grinder is a good idea, but I wouldn’t count on changing your parents’ preferences. If you don’t enjoy their preferred roast/ratio, just bring some of your own beans and make a separate pot for yourself when you visit. I had a similar situation with my in-laws and that is our solution. I bring my coffee when I visit them and they bring theirs when they visit us and we’re all happy.
I’m your evil twin in regard to raisins. I like them in savory dishes and salads, but can’t stand them in desserts and baked goods.