Well, we don't see a LOT of characters in both the 82 movie and this one get taken over by the creatures yet at some point they did. That's sort of the whole point in these types of suspense/"who did it" movies. We do lose track of him as an audience between scenes during which time he could've been duplicated.
And, again, it wasn't a helicopter. It was a land-based snowcat.
And, yes, she comes to the conclusion after not seeing the ear ring because as she said earlier in the movie, the alien cannot duplicate metals, only the creature itself. It wouldn't be able to duplicate the earring and I doubt it'd even be able to duplicate the hole in the ear. And when she asks the guy about this he even reaches for the wrong ear he had the ring in. This all clues MEW that he's a thing.
Why he doesn't change is mostly unknown, but there's a couple of times in the Carpenter movie where the thing doesn't reveal itself on death, and this could either mean we're supposed to wonder if she really killed the thing or the actual guy or that maybe the thing had "learned enough" to not reveal itself.