So, the good news is, at least you're trying to have this conversation now!
The wedding question is a good one because traditionally recognized Catholic marriage 1) must be done by a priest and 2) must be done in the church and 3) is accompanied by pre-Cana classes (assuming there wasn't a dispensation). You may already be in a very flexible place as far as the ceremonies go, so, yes, it's a good sign the family is more flexible than you're giving them credit for, if you were able to do what you described. Also, traditionally, Catholic weddings to non-Catholics include a promise (and acknowledgement thereof) that the children will be raised Catholic, through every effort of the Catholic parent. Though, if you were brought up Catholic, you may have skipped some of this.
If your husband is agnostic, and this promise wasn't part of your wedding, you have a fair reason to ask your husband why he would baptize your child. He sounds like he has no intention of bringing him/her up with the church, and the issue seems to cause you a lot of distress. All the paths from here have trade-offs.
Theological discussion may or may not be of help to you - there are plenty of fine reasons people are Catholic that aren't about fear of punishment, and no one was ever indoctrinated by being baptized as an infant and then never going to church - but more importantly, I'd like to comment on your "zero shits" statement. The only reason for anger here is to defend yourself against a feared outcome, and it sounds like you fear your primacy as a parent may be overridden by an overbearing family. You have the opportunity to set boundaries to prevent that and you have a right to do so, though it will probably go much more smoothly if you're not angry when you're doing it.
So on the "how to be calm" half, I suggest telling yourself that you're not "giving more than zero shits" just to get trampled. You don't have to be trampled at all. You may find some value, say, in networking and building family ties for yourself, your relationship with DH, and your child's relationship with the grandparents. That does not require capitulation of your beliefs, or necessarily of theirs, so you may be preparing to throw down for the Alamo a little earlier than is actually needed (if it's needed at all). This wasn't such a huge priority for either of you that you went about settling it before getting married, so that may be a strong sign that this needn't be a lose-all/win-all situation for you, or for his family. My fiancee is Catholic, and she and I talked about it, her family and I talked about it, she and her family talked about it, the priest talked about it in pre-Cana... so it's probably not as big a situation in your case as you think! Best of luck to you.