Lollie, OK, I'll try to help you.
1. Yes. There's a museum of mafia in it.
2.Michael's great love for Fabrizio? Maybe you mean someone else? Fabrizio was a traitor and Michael had him killed. Read this thread attentively: Fabrizio.
3.No, he was not.
4.Neri came to serve Michael in the end of the book, so "the guy" was someone else. I don't remember who. You may post this question in the trivia game
Neri is not too central. He is Michael's executioner, prime enforcer. His character has a lot of space in the book.
BTW, If you read it, you'll have much less questions.
5. 1-Santino, 2-Fredo, 3-Michael, 4-Connie.
6. Genco is a friend and later consigliere to Vito, before Tom. He was not a lawyer. He is in GF2. Book says more about him.
7. they went to the mattresses to protect their wives and kids from attack. They lived there because it was more convenient. Like army barracks - you don't call every soldier every day from home to the battlefield.
8. There really were 5 families of NY, some of them still exist. There are many versions, but I believe Puzo took as model different mafia leaders. Some people say Bonnano, some - Genovese, and some-Costello. There's no full resemblance.