During the poker scene John has been dealt what is known as the Dead Man's Hand. This consists of a pair of aces and eights, along with an unknown hole card, as was reportedly held by Old West folk hero, lawman, and gunfighter Wild Bill Hickok when he was murdered while playing a game.
Constantine narrates that his deaths started being comical after several of them. Pay in mind eventually anyone would start going crazy and laughing at atrocities once their sanity started to break.
Despite Constantine keeping his memories, the copies of his loved ones don't exactly resemble the old universe versions. Zatanna notably never wore a black outfit with red trim while performing; she either wore blue or a white top with black pants.
Jason Blood already shared a body with Etrigan. It makes sense they swap whenever Constantine inadvertently causes a nightmare scenario where demons kill him.
Constantine sold his soul to three demons; neither of them can claim it. Simply put you can MAKE the deal, but you can pay your end if the reward is promised to another.