I wish you hadn’t quoted that, because I just actually significantly edited it. Here’s the more cogent thoughts.
The sixth season’s really getting its act together. Alt-Desmond’s job is now clear: he’s meant to nudge people in the direction towards their constant, and thus to merge their memories. It’s not about near-death experiences as you claim here (or at least, that’s not enough): Des’ near-death experience only left him confused, but he didn’t have his purpose until he met Penny.
Nudge, though, is the operative word: like Jacob, he’s trying to put people in the position of making the right choice, not forcing them to make the right choice. In fact, in an alt-world without Jacob, Desmond IS Jacob, because he’s the only one who can see the big picture right now.
So who are the constants? Sawyer and Juliet seem like fan favorite constants, but I don’t think they’ll go this route: I think it’ll end up being Kate. Sun and Kim are likely constants, but since they’re already involved, I don’t know why it’s not kicking in. I assume it’ll all be explained in time. My guess? It’s not each other, but their baby.
What was Desmond trying to accomplish with Locke, though? My guess is that it wasn’t a hit, but actually meant to put Locke on the path to getting Jack to operate on his spine and “fix” him (as Jack also “fixed” his future wife in the first episode of season 2. Wasn’t she also hit by a car?).
I think this theory at least explains what’s going to happen in the alt-world for the rest of the season. There’s six hours left. To wrap up the issue of constants and get everyone on the same page, you need one episode each to deal with. Here’s my guesses:
Sawyer – > Kate
Jack —> Locke
Jin + Sun --> Baby Kwon?
Sayid —> Nadia? (Will establishing his alt-world constant “fix” Sayid on the Island?)
If Sawyer and Kate are each other’s constant (blech), you have another hour to play with… which I’d give to Ben. Rumor has it he has a “surprising” romance this season: my guess is its with Juliet. Is it possible that Juliet is Ben’s constant?
Then you have one hour left to deal with Jacob’s story, which we also know is coming.
For me, the big reveal will be whether or not Jack and Locke are each other’s constants. If so, the show is going to end the same way the last episode of season five started: Jack as Jacob and Smokey as Locke, sitting on the beach, watching a ship come in. It always plays out the same. Man of science, man of faith.