If the helicopter landed on its tail, it's possible GriggsThing was crushed and then burned by avgas, while Carter and Jameson were saved by their safety harnesses and managed to crawl from the wreck.
As for it being inaccessible, I believe it was Edvard, the team leader, who pointed out the site was impossible to reach and everyone just nodded their heads in shock. As he was almost certainly EdvardThing at this point, he would not have wanted anyone to investigate the crash caused by GriggsThing. He then insisted Carter and Jameson be killed the moment they returned to camp, later that evening, when right after the crash he had been downplaying the possibility of the Thing imitating anyone.
If Edvard was a Thing at the time of the crash, the survival and return of the crew is not a plot hole.