This is a sentence from the fifth story in キノの旅, which is 大人の国. For context, the narrator is talking about a time when she was a kid and she said something about the "surgery" that offended her parents. Everyone in the country has a surgery to become an adult when they turn 12.
I find this whole section pretty difficult, and it's hard for me to say exactly which part I'm confused about. I'm assuming それ refers to the surgery (and not, for example, what she said to her parents). If that seems wrong please correct me.
How should I parse the second sentence? Are みんなが and 自分達が part of the clause that ends with 無理矢理された? Are they the subject of 思い込む? Does 自分達が無理矢理されたこと mean "what was done to them against their will"? And why does the sentence end with the dictionary form 思い込む rather than something like 思い込んでいたのだろう?
Sorry for asking so many questions! Feel free to just answer whichever questions seem most important.