The question about the "main scenes" was a little confusing. I think you are actually talking about settings, in which case I would argue that Las Vegas also deserves a mention.
Not sure Luca Brasi is "seemingly invincible" either. He gets murdered almost immediately.
According to some of the behind the scenes blogs people write, even Don Corleone was somewhat afraid of Luca. He supposedly killed his wife in a fit of rage before he ever met the Don. You can tell that he wasn't "a smart man" as Forrest Gump would say and he got caught off guard when he met with the Tatallias.
I think "horse head" should be accepted for that question, and if I remember correctly the drug business they were getting involved in was stated to be narcotics so you should accept that as an answer for the drug question.
Not sure Luca Brasi is "seemingly invincible" either. He gets murdered almost immediately.
Good idea for a quiz though.