Staying strictly within the confines of the original film, and not taking in to account any additional material or paratext, this is how I've always felt:
1) Speculating on how clever The Thing may be and all the different things it may do to convince its threats is fun but not textually valid. It's inherently speculating on things that aren't depicted and don't happen and while they could happen in any infinite possible different universes there is nothing in the movie to suggest that.
2) Speculating on how clever MacReady may be is no different. Stuff like "maybe the bottle at the end had gasoline in it" isn't really intelligent insight, I don't think. That doesn't mean it's not fun to do (this channel actually understands this quite well and I enjoy them for that reason), but I just don't think it's smart speculation.
3) I am inclined to lean on the cinematographer's "eyeshine" as a strong indication of who is or isn't The Thing because this is a deliberately professed storytelling tool. We have the "word of God" on that. However, you always have to be careful with this. Just because something was intended doesn't mean a movie can't have continuity errors or mistakes.
So, for me, the most important thing (ha) here isn't really figuring out the puzzle so much as how the ending relates directly to the movie's theme of paranoia. I think there are meaningful decisions made in how the final scene is portrayed to make you think Childs is The Thing. The fact he drinks from the bottle, the musical queue when he does so, the fact MacReady laughs, and the lack of the eyeshine, all suggest that Childs could be The Thing.
But you don't know. You can't know. At most you can suspect. That distrust and paranoia is what the movie is really about. So ultimately whether Childs is The Thing or not isn't what matters - it's that you think he could be and so does MacReady. That's the full depth of the ending. Not about giving you an answer, but not giving you one.
That said, my current interpretation of the ending is the ending is that neither are The Thing.