I wonder if some of your answers would be different here if they had read your SWH first.
Your inlaws are the biggest losers in this. Your husband feels differently about his parents because they are racists. They don't see their grandchild as much as they would because of their racist views about his sister. Your son won't be as close to them as he might have become BECAUSE they continue to foster this us versus you demeanor, and they take it out on your daughter. They refuse to apologize even though they have treated all of you badly because your child is of a race that they didn't want to have in the family. They now pretend that they are treating you well. They aren't. They rub into your face that they are supporting their son's child, but not yours BECAUSE OF HER RACE. For those who think it's just because she's not their biological granddaughter, they made it perfectly clear to EVERYONE that they didn't want a bi-racial child in the family. They still don't, so they ostracize her by demanding that she not call them the same name as her sibling. THAT IS WRONG.
The college fund is a different thing. I am not defending it. I am saying that there are plenty of people who don't do this for kids who are not actually kin to them. It IS rude and cruel of them to continue to write in the card your daughter sees when you have already asked them not to do it.
Truthfully, they probably aren't going to let you all see the bank account because this is a way of controlling their son.
I truly think that the best thing that your family can do is to visit them together. If they bring up to your daughter that she can't call them a certain name, your husband should simply say "It's time for us to go" and then leave. If they want to see their son and their grandson, they will have to behave.
You will NOT get an apology out of people who aren't sorry for their behavior. They will not love you or your daughter the way you want them to. The rest of the family will continue down the path they are going, and you and your own family will need to just continue down your path.
When your son starts asking questions, you'll have to decide what to say. I think that you and your husband should ask the counselor how to have that discussion before the questions start coming.
Stop wanting things between you and them to be different. You and your husband need to just stand strong on what you decide to NOT allow from them. The name thing is absolutely inappropriate and any time they start it up, that signals time to leave, hang up the phone, etc. Foster love between your children and don't allow your inlaws to hurt their relationship with each other.
As to your question of whether or not you are making this bigger than it is? No, you aren't. However, you are expecting pie in the sky if you think that they will EVER say they are sorry. You are expecting pie in the sky if you think they'll ever love you or your daughter. Quite honestly, if you are nice to them and act respectful, yet draw a line when they are hurtful to your daughter (along with your husband), then you are in the clear and in the right. You and your husband have every right to have boundaries that protect your nuclear family. And those boundaries include spending less time with them when they continue to marginalize your child because of her race.
Until your husband CONSISTENTLY tells them that intentionally hurting HIS daughter (and he should call her that to them) is inappropriate, and ends the visit each time with all of you, they will not have any incentive to behave so that they can see their son and grandson. I hope that he will have the tenacity to do this. It will speak VOLUMES to both of the children when they hear daddy taking up for your daughter. Your son will also know that you two will take up for him whenever the time comes that someone is cruel to him. Don't for a moment think that the thought wouldn't enter his mind "My parents let grands treat sis this way. I wonder if they would let someone else be mean to me..." Kids are pretty smart.