So following this week's episode of Lost, it would seem that I was not only wrong, but that the relationship between the regular Lost universe and the so-called flashsideways universe is the EXACT OPPOSITE of what I had guessed after the season premiere. Oops.
That is, it looks like the alternate reality in which the Losties never went to the island takes place, in our story, before the ongoing saga on the island. And that Desmond, having gained a vision of the reality that was erased when Juliet set the bomb off, is going to convince them that the universe is wrong and they need to (somehow) reestablish the previous one. So, presumably, at the end of the series they'll choose to go back, ending up - diegetically, if not temporally - back where we began the season in the main universe, with our heroes in the crater, not realizing that they had succeeded, only to undo what they had already gone and done. We might even learn that the Man in Black was unleashed in this alternate reality, that he's given everyone what they (thought) they wanted - Hurley is lucky, Desmond has Widmore's respect, Jack is the father he wanted his father to be - and that they'll have to reject it in order to save the universe.
So here's my proposed order for how things unfold:
1) Juliet hits bomb and sets it off in 1977.
2) Island implodes/explodes, but does so with enough time for Chang and Widmore, at the very least, to escape.
3) MiB is freed.
3) The Losties get what they want from MiB in the ensuing years. This is an important difference from my earlier supposition, which was that they get a happy ending. Jack gets to be a good dad, and this is happy; Desmond is Widmore's right hand man and has his approval, which is a bit less obviously happy because he doesn't have Penny; but it also looks like people who are carrying a lot of guilt - Sayid, Kate, Sawyer - are being allowed to punish themselves, which is decidedly not good.
4) Desmond will convince the Losties that what they want isn't as important as what needs to be done. They'll restore the previous timeline, somehow.
5) Everyone wakes back up where they were in 2007, when the season premiere began with Jack and company in the crater.
So that ties things up. I'm not sure that I like it - that there needed to be an alternate reality where they never went to the island, especially when it feels like the wind-up might be unsatisfactory in the mainstream universe - but at least it hints at the ultimate meaning of alternate reality. And it made this other universe appear to have a purpose, which it had been lacking for a few episodes.