Seriously WTF. Why wouldn't Emmett Brown say to Marty 30 years later: "Sup man, you were in the past, you actually gave me the idea for the time machine and saved my life with that letter"
And why wouldn't Lorraine think when her child had grown up like: "WTF this guy looks exactly like Marty, the guy I liked 30 years ago and suddenly disappeared."
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For your first question. Doesn't Doc get the idea for the time machine after he falls of a ladder? This occurs shortly before he meets Marty (hence why he has a bandage on his head when they first meet) and therefore he knows his idea works. Therefore he already has the idea.