On the same site you linked to (Outpost 31), there are interviews with John Carpenter and Stuart Cohen (the producer). In both of these interviews, both men are repeatedly asked to disclose when various characters were assimilated. The answer to every single one of these questions is the same: "We don't know."
There are any number of fan-created theoretical timelines of infection, but none of them are any better than the others, because there is no correct answer to this question. The director and producer deliberately avoided setting things (no pun intended) in stone, and Carpenter genuinely shows no interest whatsoever in the issue. He doesn't know, and he doesn't care.
Cohen says more than Carpenter, however: in his opinion, Blair was assimilated sometime between the autopsy on the two-headed Thing and his tantrum in the radio room. But, just like the fan-created timelines, this belief is no closer to the truth than any other, for the simple reason that there is no truth. People were assimilated whenever you think they were assimilated.
This is probably my favorite movie ever, and I've probably seen it 20 or 30 times. I've read the novella upon which it was based (Who Goes There? by John W. Campbell), I've read the short story inspired by the movie (The Things, by Peter Watts), I've read every interview I can find and watched every video of every Question and Answer panel Carpenter and the rest of the cast and crew have done, and I've even read all the draft scripts available online. I've also seen the mediocre prequel several times, although I don't particularly enjoy it, and read interviews and drafts of the script for that as well.
As much as I love The Thing, and as curious as I am about these kinds of questions, I don't try to figure out all the details, because - as you have already said yourself - the whole point of the movie is the crushing sense of uncertainty. I would assume that the reason Carpenter and Cohen never determined when each character was assimilated is because, if they did so, it would create an absolute certainty - precisely the thing they were trying to avoid. If the people who made the movie don't know when each character was assimilated, no one can ever know.
And that, quite simply, is what makes this film such a masterpiece.