Yes. I agree. Jack became just a part of Lureen's life-plan - a husband, a father, a parner in the business - but I don't think she was involved with him emotionally to any great degree after the first few years - her ties to the family, particularly her father, were too strong.
Alma, had Jack never come along, would have stuck doggedly with Ennis, I reckon, looked after the girls, got herself a job, maybe pushed ennis into something better-paid and more steady - Alma'a pretty conventional and wouldn;t have gone through a divorce unless there was some positive reason.