After intense discussion, regarding the original, there's no doubt in my mind that Palmer was the first human to be assimilated at the American camp. The room that the dog enters was Palmer's bunk room that he shared with another crewman, but I don't remember which. The slim figured shadow on the wall also matches Palmer better than the heavy-set Norris. There are several scenes later on in which Palmer is actually missing from the group. I think when they are gathered around outside burning corpses, about two or three of them are not there, but I don't remember exactly who.
Norris was assimilated much later, though it is significantly debated whether Norris was quickly and violently assimilated or if he had gotten a small infection and was slowly overtaken, causing his heart to collapse and the infamous Norris flip-out with the defibrillator. (Fuchs suggests that everyone start eating out of cans because of the possibility of infection via ingestion) We remember when Gary offered leaving Norris in command, after he nearly shot Windows for flipping out, and he says very soft-spokenly, "I'm sorry fellas, I'm just not up to it." If he was an imitation at this point, this is characteristic of the Thing not wanting attention put on himself and preferring to be in the side-lines. What pissed me off about the prequel was that the Thing did not do this, it simply attacked everytime it had an opportunity without thinking about how many people were near it. The helicopter Thing-out sequence was incredibly ridiculous because of this reason. It would never try to assimilate all of the people at once and sabotage its free-ride straight to a more populated area. The Thing in the '82 version was not like this at all, it preferred working at a very slow pace and always deflected attention from itself by acting normally even sometimes in intimate scenarios to achieve its goals.
The thing we should remember is that the Thing had ample opportunity to take over Clark the dog handler, Windows alone on the radio, and Mac who lived outside in his own shack. But it didn't. It took the most low-key people who access to knowledge of the surrounding area. Palmer was a mechanic training to be a pilot and Norris was the geologist. Blair of course, completely snapped, when he discovered the extremely high likelihood that the organism he was studying had already infected a member of the crew, maybe even himself and what it was capable of if it reached a more populated continent. (The prequel makes only little mention of this possibility really) Blair presented a significant threat to the Thing because he had the absolute best intuition of the crew and had the scientific knowledge of organisms to understand what it was capable of doing. (Kate really didn't in the prequel and yet figured it all over the course of a day, where it took Blair days to figure out what was going on)
It is unknown exactly when Blair was infected, most think that he was infected when he was isolated in the shed, but some think it was actually prior to this.
Regardless, when Windows the radio man, happened upon Bennings the meteorologist in mid-assimilation from the Split-Face, he panicked and dropped the keys that Copper and Gary owned. These keys were picked up by Blair, who was a Thing by this point, and sabotaged the blood supply shortly after Copper suggested making a blood-serum test. Everyone else was accounted for.
Fuchs' death (Doc Copper and Blair's assistant) was not suicide as he did not have access to flammable material to set himself on fire. Not unless someone discovered his corpse before Mac did and set fire to it. Fuch's most likely cause of death was murder by another human and burned. A thing would have no reason to simply kill a human without assimilation when alone. Fuchs was freaking out when he glimpsed someone pass by after the power got shut off. The person he likely saw was probably Norris or Blair. Shortly thereafter, he runs outside and discovers MacReady's shredded clothing, insinuating that Mac had been assimilated. He probably freaked out and tried to find someone, only to be shot and burned because he startled them. The lights were out for over an hour (according to Mac) and everyone was split up at the time. It could have been anyone.
The question of Childs' or MacReady's humanity is the most contentious one by far and it is 100% subject to one's own interpretation. I don't see any reason to believe that Childs is human when he 'gets lost in the storm' looking for Blair. Shortly after Childs wanders off, the power goes out yet again and the generator is completely destroyed. (When Mac and Gary go to the generator room, Gary laments, "It's GONE MacReady") Also, by this sequence quite some time has actually passed since the blood test scene. Mac could very likely have been assimilated at any point in the final sequence of the film. I have reason to believe that some Thing imitations will sacrifice another for their own benefit, like Palmer-Thing did with Norris' head, when he sabotaged its escape. "You gotta be

in' kidding!" So the idea that a MacReady imitation would kill the massive Blair/Gary/Nauls/Dog creature it morphs into is not entirely unplausible. I like to imagine that the final dialogue at the end of the film is actually two things talking to each other talking about what they should do next. Creepy and eerie.