I guess Rockwell's win is so taken for granted that no one's felt the need to add much here, but I think you make a couple of interesting points...
...especially about Richard Jenkins. It's rather remarkable that he's shown such strength as a nominee -- getting in at SAG even while the film was missing Ensemble -- yet has never for a second seemed a potential winner. If there were subterranean opposition to Rockwell -- which I don't think there is, but assume -- Jenkins could be the kind of "we didn't see it coming but why didn't we?" upset that Anna Paquin was over Winona Ryder, or Kevin Kline over Martin Landau.
But Dafoe seems to have sucked up all the possible upset energy, largely because of that critics' run and, it seems to me, the fact that bloggers seized upon him at Cannes and just locked into the idea. Like you, I just don't understand where this enthusiasm came from (this one, to me, is the "manufacturing consent" scenario), but, because of it, Dafoe remains at least a possible surprise win.
A brief moment to say Christopher Plummer has had a worthy career with, now, an interesting series of late career fireworks, and he's perfectly fine in All the Money..., but this is an utterly unnecessary nomination that I'll always feel robbed Michael Stuhlbarg.
I understand what you're saying about Woody Harrelson -- I felt/expressed some of it when I saw Three Billboards, calling him the soul of the film. It's a beautiful performance, and, given the man's surprising longevity (and, now, three nominations), you'd think, like you say, he should be in the race. But 1) Despite his being in the public eye roughly as long as Dafoe, I don't think he's held in near the same esteem, for a reason you articulate: he came to us from TV, and not just TV: a sitcom. Yes, it was an Emmy-winning sitcom, but that was back in the day when the Emmys thought L.A. Law and thirtysomething were great dramas. Harrelson has spent much of the three decades since striving to reach respectability -- I'd say he's definitely there now, but it wasn't always the case. Whereas Dafoe was at the top of the respectability ladder after Platoon/Last Temptation, and has been accumulating extra points since. 2) I had the same feeling about Harrelson that I did about Tommy Lee Jones in No Country for Old Men -- beautiful performance, soul of the movie. And in both cases my next thought was, yeah, he should be awards-cited...but there's that other guy in the movie who just has a clearly more dominant role/performance.
I know we disagree on Three Billboards, and I think it's come to the point where fans/foes of the film almost can't talk about it (I've been a long-time fan of Mark Harris, but he's annoyed the crap out of me this season with his Three Billboards sniping). For those who disIike the film, Rockwell's character/performance is seen as the root of the film's problems. But the flm's fans -- and, per SAG/BAFTA/Globes, there are quite a few of them -- don't have this issue with the film, so they simply see it as powerful, complex, dominant work in a major best picture contender. I'll grant you that Rockwell's career-points kicking in here has been something of a surprise -- he's always been an actor the cognoscenti have known, but you wouldn't have thought his resume would be so familiar to so many groups as to help carry the day. But apparently a long-time reputation as dependable character man has done more for Rockwell than we might have thought up till now.
I honestly see him as perhaps the most certain of the four seemingly-locked acting winners...which may just mean I'm going to be shocked tomorrow night. But I feel pretty good about betting on him.