"Deus ex machina" plot twists give me headaches.
Not to mention being the mark of either amateurish writing or an author who's painted him/herself into a corner.
IMO how Ennis would react to Jack returning would depend on a couple of things, especially -
1) Why has he been gone? If it's something not under his control, that would be one thing but if it's just a deliberate disappearing act that serves as an ultimatum, well....
2) What Jack would have in mind in terms of living arrangements. After his history with Ellery I can't see Ennis going back to just seeing Jack in a wilderness ghetto once or twice a year.