I cannot stress enough how J_hn W_nchester being in Heaven proves that the finale wasn't real/was more of Chuck's writing.
J_hn and Kevin got out of Hell in nearly identical ways (J_hn through an actual gate, Kevin through a giant crack). We are then told that souls from Hell cannot go to Heaven, unless God makes an exception like in Bobby's case. This means that J_hn's soul should not be in Heaven. Yes, Jack supposedly took all of Chuck's powers, but how then would J_hn be in Heaven? Jack would theoretically know and understand all of the pain and suffering J_hn caused Sam & Dean, so Jack wouldn't bring him to Heaven since Jack wouldn't want them to have a constant reminder of their abusive past.
But do you know who would want to make the Winchester's Heaven into a living (dying?) Hell?
Chuck.
Chuck has dealt with them for such a long time he isn't going to let them off easy. How is it that Dean would die on a piece of rebar when Jack, his son, is now god? Simply put, it just wouldn't happen. Chuck would love to watch this play out. It is already shown that Chuck loves painful goodbyes before some is meant to die (shown in 14x20 when he tries to make Dean use the Equalizer to kill Jack).
Chuck won my merging with Jack (against Jack's will) to become Juck (Chack?) . Jack is currently trapped within Chuck's mind and is trying to get out, in a similar way that Dean trapped Apocalypse!Michael within his mindscape.