I think you are spot on. My feeling is that Alma's desire to see or even confront Jack was totally irrational and not pre-meditated. I also had a feeling that she had no idea what to say anyway. But to understand Ennis at that point, she needed to know more about Jack, and the only wat to do this was to see him face to face. Perhaps Ennis knew this also, and that was why he was intent to keep Alma and Jack separated.
I don't think that a woman in this situation can do anything but irrationally; we are happy by seeing Ennis and Jack's happiness,but as many times in life,happiness is built over someone else's anguish and sorrow.And it's obvious and doubtless that-even if some of us have seen Alma as an "enemy" of our boys sweet life-a woman who loves deeply his husband and discovers in such a brutal way that he's cheating her -with another man,moreover-,can't react in a pre-meditated way as all her world is breaking.
So,I have always thought that her first and main intention was
trying to understand and after that,trying to know in what she had failed-something very usual in women in this situation...-; if her rival had been another woman,she had known how to fight with the another woman's "weapons",but a man,in a rural and inarticulate woman of the 60's,was too much as to be able to make a planning and fight for her love...As you Thomas,say,she needed to know more about Jack to have an answer to her questions; if sooner or later,she had confronted him,it's something that we can only suppose or imagine.
But,surely Ennis saw this possibility more clearly,although he didn't know for sure that they had been seen,and,to avoid it,he wanted to separate them.Letting aside the fact that there have been too many years without one another; too many words to say,too many "things" to do

,too little time to be together...Knowing Ennis or not that Alma knew it,the most logical is that he didn't want to share Jack more than in the way he was obliged to share him...