And still, I can't manage to agree with you. Here are some reasons for that, for you to pick apart at your leisure. 
I’ll attack it with a vengeance.
When he comes up unannounced after the divorce, Jack tells Ennis he "got [the] card the divorce came through". "It came through" - that means he knew the dicorce was in the works, didn't he? That's how I understand his line. I'm not certain of the process of divorce in Wyoming in the 70's, how long it would take from Alma filed till the divorce was granted, but I would guess it'd take a longer period of time than from the last time Jack and Ennis saw each other (which likely was not more than 5 months?). If, at their last meeting, as seems likely from Jack's words, Ennis told Jack that a divorce was in the works, *that* would have been the time for Jack to think that the Alma obstacle to their "sweet life" had been removed. He'd have no reason to curb all his enthusiasm till the divorce actually and finally came through.....
Oh, my. I do disagree. Not at all with your argument… but your conclusion. I don’t have any problem with the way you laid out the events. Could be. But, I disagree starting with the word “that” in asterisks. No, I don’t doubt it would have put Jack in a perky mood to hear of the divorce in the works, but, because of how long divorces can take, Jack would have still been on the wait for the actual final decree. After all, that is when he showed up. He probably didn’t curb ALL of his enthusiasm, but that doesn’t take away from the fact that it was the note from Ennis that got Jack to drive up there and still be quite perky.
Additionally, when I think about it, had Ennis told Jack *in person* about the pending divorce, on one of their trips, would not Jack have said something like “sweet life” at *that* time; thus, wouldn’t Ennis have dropped the sack of potatoes onto Jack *then*? But he still came up looking all perky. He had expectations -- expectations that hadn’t already been dashed.
This tells me that there must have been something else in Ennis's note, something more than a flat "I'm divorced" statement. Don't know what that may have been - but it was something that must have led Jack to think, erroneously, that not only Alma but also the Earl obstacle was out of the way. The white truck and Ennis's immense discomfort then brought him crashing down to earth. 
First, it’s very possible that Ennis’ note said: “Hey Jack, Alma filed for divorce last April and it just came through…” So maybe he didn’t know anything about it before that one card.
Well, I can go with Jack thinking that both obstacles *were* out of the way because I argued that Jack was unaware of the deep significance of the Earl excuse to Ennis’ psyche. Whether Jack was actively thinking about the Earl excuse or not doesn’t seem to be a factor since Jack learned more about its depth when the white truck drove by.
Yes, Jack is a dreamer, but not so much so that he isn't aware of (and adapts his public behaviour to) the codes and rules that apply to the time and place where he lives. He full well knows there is a *huge* hurdle for them to overcome in trying to "ranch up" together. Stories like the Earl one can't have been one-of-a-kind, unfortunately, and Jack grew up under the same conditions that Ennis did. He can't have been unaware of the danger of physical violence and of social contempt and ostracism for anyone being perceived as "queer".
Yes, I can go with this. But, Jack didn’t experience the Earl story as Ennis did. First-hand is much stronger than second. Also, Jack was more of a risk-taker. And by this, I don’t mean that he would see danger and bite his thumb at it. Rather, he would be more willing to take the risks of social contempt and ostracism.
In fact, IMO we see him demonstrating the awareness of this several times: In his uncomfortable reaction to Jimbo and those people Jimbo goes to talk to.
To me, his initial reaction to Jimbo’s decline was more of “Damn!” than of fear of blowing his cover. After all, he didn’t hightail it out of there instantly, like a scared little rabbit. He lingered and watched Jimbo walk over to the other guys and lingered a bit more and didn’t actually leave until the bartender made the calf roping comment. It seems to me that that’s what pushed him over the edge. Disgust at the bartender’s snide remark. Of course, coupled with the fact that he just "outted" himself.
In the way he (as far as we know) keeps his gay activities well away and invisible from the community where he lives (going to Mexico, going up to Wyoming).
Yes. But, he’s not just hiding a gay affair – which, for him, in the time and place, certainly required a big invisibility shield – he’s also hiding an *affair*. He would have probably acted the same way had his extracurricular activities been with a woman. For example, had Ennis been Denise, and lived in Wyoming, Jack would have traveled… to Wyoming. Besides, having a gay affair when one is married is something to be very careful about with regard to one’s community. Regardless his level of “gay bash concern,” his travel to Mexico is reasonable.
And in his story about "expecting to get shot when he goes to see the ranch foreman's wife".... I know this is only my interpretation, but hey - IMO, Jack is adapting the real story to fit the talk with Ennis there, but he's drawing on real events and real concerns. To me, what he's really saying is that he's got an affair with Randall going (the first affair so close to home and so close to his local community) - and he's aware of a real danger of physical retaliation from the community associated with this. (Which IMO in the end also did in fact cause Jack's untimely death, but that's another discussion!) Yes, I know - doesn't mean he's scared enough by all that to not continue seeing Randall anyhow - but at least he's entirely *aware* of the risk and so wouldn't make lightly of Ennis's fear, especially when he knows Ennis has been so directly subjected to witnessing an actual gay-bashing in his formative years.
Yep. No doubt. Plus, Jack tenderly rubbed Ennis’ ear after asking “You seen this?” Jack was the comforter. He knew of the dangers and that the Earl story would have *an* effect on Ennis. And, I’m sure Jack always knew that the effect on Ennis would be greater than on Jack, himself; but, Jack still don’t know the exact depth of damage that had been done to Ennis at the reunion river scene. He learned more and more about it as time went on – as did we.
I understand that your "Jack is a dreamer" aspect doesn't mean he's not aware of any of the above, but that he optimistically thinks that "amor vincit omnia". Still on the background of all the above, however much he thought he and Ennis could overcome the ghost of Earl (and I agree he obviously *did* think that) I find it hard to believe he would *underestimate* the Earl story's significance once he heard it, underestimate its devastating impact on Ennis, when listening to Ennis's tortured account of the event.
Right, as to the whole first part. But, as to the underestimating part… this is on a continuum. On one end, Ennis has NO fear; on the other end, Ennis has ALL fear. Jack’s understanding wasn’t all the way to either end. It was somewhere between the two polar opposites. And it was more than likely closer to the ALL end than the NO end. But, the behaviors Jack displays and the fact that he suggested that Ennis move to Texas – as if that would be any safer – tell me that Jack’s estimation is farther from the ALL end than is Ennis’ actual placement on the continuum… very near the ALL end.
And moreover, Jack's the one that understands the taciturn Ennis best. I'm not implying he understands all, as he demonstrates later on in his "I'm not you" accusation, for instance, but he *is* the one who is able to draw him out and make him open up a little.
Are you saying he’s good with a can opener?
He's the one who's best able to understand that when Ennis eventually comes up with this story and uses more words than he's used on much else it's not because it's just "one more thing" - but THE thing - the big thing that Ennis hasn't brought up till now, because it is so painful, so raw, and so connected to what and who he has so far not wanted to admit to being - even to Jack, even after they started having sex.
Well, this is one valid way to look at it. However, it’s also valid to think that Jack didn’t get the whole picture from Ennis -- that is, Jack may have been confused about the significance due to all of the reasons Ennis was giving. It wouldn’t be the first time (or the last) that Jack misread Ennis and was confused about exactly where Ennis stood.
The only reason he seems to manage to tell it at all, after having shielded his eyes thoroughly with his hat, is because he needs to shut jack up from further arguing by giving Jack the real reason …
Sure. To Ennis. And to us. But to Jack, just another reason. This is consistent with the behaviors Jack displays later in the film regarding his understanding.
At the whole Alma obstacle he scoffs "You and Alma, that's a life?". When learning of Earl though, he is schocked "You seen this" (According to the script he goes white and can hardly believe what he's hearing, feeling as if he's in a free-fall. I think HL is better than JG in going pale when the script demands it..........)
No doubt. (Side note here that may get me in a heap of trouble: Heath EARNED the Oscar. Jake earned the nomination.) I don’t mind going to the short story or script when it’s consistent with the film. But when it’s inconsistent… Honestly, Jack does NOT look like he’s in a free-fall. Even after he heard about Ennis’ parents’ deaths, he said “Shit. That’s hard.” Here, he just gave a little ear rub. How precious was that?!
Ennis is obviously deeply emotional over this,
Sorry. This made me LOL. This is kind of funny. In a bittersweet kind of way.
…about as emotional as Jack ever sees him till their last meeting - and Jack does seem speechless at what Ennis has just told him. At least, he seems to completely understand that the true obstacle is here, out in the open - and he doesn't protest any more because he sees that he can't fight this, not at once. It will take comforting, and calming down, and time........
I’ll agree with the “as emotional” part… somewhat on “speechless”… but his behavior in the post-divorce scene seems, to me, to contradict the rest. I have to stick with the idea that Jack heard the Earl story, understood that it was deeply emotional for Ennis, but did not understand its true weight, and thought of it as something that he, himself, would be able to overcome (if it were his problem), and thought of it as something they, together, could overcome with Jack’s help, reassurance, etc. – over time. But as the film progresses, Jack sees it’s not able to be overcome. And this reflects the notion that he’s also getting clued in more and more throughout the film about the depth of its impact.
So, in my opinion he does understand what an enormous hurdle Earl's death is to his and Ennis's happiness - how hard it will be to overcome.
OK.
Jack *does* see it for its huge significance to Ennis’ entire being.
I can’t go this far.
Based on his understanding of their time and place, based on his knowledge of Ennis, based on his reading of Ennis's emotions and reactions.
Even with all of this and a slice of apple pie, too.
So however happy he is that the divorce came through - I just can't see him disregarding Earl to the point of thinking Ennis would be immediately ready for ranching up because the marriage was over.
Agreed. 100%. Jack was… oh, oh… I’m going to get myself in trouble again… Jack was, well, a bit foolish – in the post-divorce scene.
Except.... that is what we seemingly see him do. 
As you've eloquently pointed out.
Oh, no. I have never believed that Jack was being realistic in the post-divorce scene, if this is what you’re referring to. No. Jack was aware of other obstacles. And he knew they’d have to be overcome. But, for those seven years he was so focused on the marriage reason that … well, he acted foolishly. (Oh, I know this one is going to come around and kick me in the butt…)
