I know this is a very tough time for you, but you need to think rationally. A man who can give a sudden divorce,without following Islamic law and that too without giving an ounce of respect to the mother of his child can truly care about his daughter? A man who can even think of separating a mother and a child can care about his child? No No. Most likely his family just want to keep her because she’s blood related. They will raise her with the same morals they have within themselves, which obviously aren’t so great.
There are many questions which are unanswered, and for most of them, you’ll never find an answer. However, the custody of your daughter SHOULD NOT be a question. I feel hesitation in your above post, which scares me. You need to make it very clear to YOURSELF, YOUR FAMILY, and if your ex-in-laws ever contact you, THEM; that your daughter will always stay with you. It is a huge mistake that they keep her.
I can tell you right now, you will never get answers to ALL the questions. Because for some godforsaken reason you ended up in this family, who doesn’t care for anyone else but themselves. Further, I hope you’re ex husband comes to his senses and explains this situation, but that is highly unlikely. And until you understand that, your healing process will not start.