Ennis feeling for Jack run deep---but unless Jack was in a coma and just regained his life back, and his emotions would be back where they left off----I am thinking that all the changes that occured between Jack's leaving and them finding each other again---would make them more friends than lovers in the present. It is possible that when Jack sees what Ennis has now with Ellery, he won't really want to interfere, and will go on without Ennis, because Jack always had that capacity. It is a hard question, and would be harder to see actually having to be worked through.
Ennis leaving Ellery who loves him so much to go back to Jack, would make him just as guilty as his ending up in a divorce from Alma because he loved Jack---too much baggage, he will end up broken.
And then there is the anger factor---as in WHERE, WHAT, WHY that will be a major thing to be worked through too.
I feel he probably would have to try to be with Jack IF Jack hadn't changed too much in the time lost--maybe jack is as much a different person as Ennis is right now, and Jack doesn't want to go back there? IF Jack moved on too---it would easy for Ennis to stay with Ellery.
But Jack has first dibbs--and the big problem I see is IF Ennis goes back to Jack, and it doesn't work out in the day to day---He'll never get the same back with Ellery either, so could he leave what he knows he has now to try to see what he might have, and if he does, he may really lose all the way around. Too tough to think about in real time. Lots of hurt either way.
One thing I would really be up in the air about---Would Ellery just let Ennis go because he loves him and knows how he feels about Jack? OR would he fight hard to get Ennis to stay with what they have, and try to convince Ennis he is better off staying with him than leaving? After all, Ellery has never been in love like this before either--and has never had a relationship worth fighting for before-Ennis is his Jack--and somehow I don't see him just letting Ennis leave without major repercussions with in Ellery. That much hurt and loss after everything else he has lost---might really end Ellery, who isn't nearly as hard emotionally as he likes to pretend. This question gives me the willies. YEEWWW, don't see anything but misery in it.
Personally---I think he should stay with Ellery.
I loved him with Jack, but with the changes--I think Ennis is better off where he is right now than trying to go back.