If the whole things sounds hard to follow, let me only partly reassure you that it is and it isn't. Yes, figuring out everything that is happening is highly unlikely on a first viewing, but it doesn't really matter. You know that Dave and John are in danger, you know roughly from whom (or what), but the surrounding details are what may take days to nitpick as only a nerd can.
Coscarelli has been dubbed one of the "Masters of Horror," but it's not quite true - he's the master of really strange comedic twists on horror premises, from Phantasm's creepy midget-making mortician from outer space to aged Elvis and JFK fighting a redneck mummy in Bubba Ho-Tep. Here, he starts from a strange twist and unleashes his id to pile them on - yes, it's based on a book, but a book that fits its newfound filmmaker to a tee. It's hard to set out to make a cult movie, as they usually set off a bullshit alarm triggered by the worst kind of self-awareness (see The Lost Skeleton of Cadavra for one such movie that'd be great if it lost about 30 minutes of the deliberately corny dialogue its director is too in love with), but I don't believe Coscarelli sets out to be weird. He makes stuff that entertains him, and it just happens to be bugfuck nuts, which is why it endures. As with his prior career highlights, I imagine he'll be fielding sequel questions on John Dies so long as he continues to work.
John Dies at the End opens in theaters today.
And if you haven't seen Paul Giamatti's attempts at reading the male lead roles in Twilight, You've Got Mail and Magic Mike, you need to.