Graveyard scene: Hes NOT crazy
The graveyard scene in the finale was obviously meant to make viewers (and Sam) question whether he is actually in a coma or just crazy. The problem is that it does not work; he is definitely in a coma. If he were simply crazy, that might explain a lot of thing, but not the presumably tremendous amount of knowledge of the future that he has attained over his 30-something years of life.
To believe that he just made it (all!) up would require believing that he is the most scientifically and artistically brilliant person in the history of the world. That is the only way he could have invented things like the notion of iPhones, Britney Spears (we heard her song Toxic), Margaret Thatcher becoming PM, and so on. Yes, brains invent a lot of stuff when we dream, but the massive number of details he would have known would be absurd (remember he knew who won the horse race).
That said, there is one, narrow scenario that could potentially work for the crazy theory: he did not actually know all the things that he has learned about the world from 1973-2006. Since viewers don’t have no access to his mind, we cannot know what he actually knows or not, so we could theoretically believe he’s crazy. However that would still be a stretch because Sam is aware of what he knows. In his place, I certainly would wonder how I came up with such a quantity of supposedly invented, yet detailed knowledge.
(Plus, if he really were just crazy in 1973, then he should jump at the opportunity to make a killing with all the future knowledge he has.)