Very interesting theories about John's possible fate at the end of the game, but I think you underestimate a characteristic of the game: that the game develops in such a way that it leads to several alternative endings, depending on the choices previously made by the player.
Taking this possibility into account, it would not be at all strange if all those deserved outcomes that you mention for the diabolically twisted John Heavens come true and appear at the end of the game, but faced with such a possibility, it is also very true that other alternative endings could be developed, some of which could certainly be bittersweet for all of us; some could even be really bad endings for the MC and/or Nicole.
However, thinking about the ending that John really deserves, I personally would like him to go to prison and there suffer a quick death at the hands of a prison bully who fell for one of his previous manipulations and took revenge, before his remaining contacts can provide him with a quicker release from prison and the chance to exact revenge on MC and Nicole.
Nevertheless, such an ending could result in Nicole losing too much if John's dirty dealings come to light (Magic Grand could go bankrupt and result in heavy settlements for John's victims, ending Nicole's stable source of income, which in turn could even cause her to lose her own home, etc).
Another possibility is that John was seriously injured at the end of the game, which reminds me of a scene from a TV series I saw a long time ago...
Could it be too much of an outcome for John? Or perhaps such an ending might be more suitable for Beeman?