Interesting question, KitKat.
If Ennis hadn't met Jack then I think there's every chance that he and Alma would have stayed married for their whole lives. Ennis would likely have taken a job that would have got them out of their financial hole (as opposed to staying in jobs he could quit to go off with Jack) and even if either or both of them realised that what they had wasn't true love, there was certainly affection there in the early days and without Ennis's feelings for Jack or Alma's knowledge of those feelings, their marriage would probably have remained as happy as most, especially given their love for their daughters.
However....I don't believe that Ennis would ever have been truly happy with Alma. Even if you're in the camp that believes that Ennis was "only queer for Jack" then something would still have been missing from his life because the person he was meant to be with wasn't Alma. If, like me, you believe that Ennis was gay but just kept it all buried deep down inside because of what he saw as a child, then that hole inside of him may have got bigger as time passed - I doubt he would ever have actively sought a solution to it, but who knows?
As for Alma, I have no doubt whatsoever that she loved Ennis, even after what she saw in '67. The final straw for her came much later (when she realised that they didn't go up there to fish) so I honestly feel that, with no Jack on the scene, she would have stuck with him.