[Elladan and Elrohir]

It is my firm belief that no one would be able to throw the Ring into the Cracks of Doom upon actually arriving there.

Tolkien writes that the Phial of Galadriel gave no light here, because all it was the heart of Sauron's realm, and all other powers were subdued.

There is no other place where the Ring would be so powerful as at the place where it was created. In that place, its will would supersede even the will of iron.

So neither Sam, nor anyone else, would've actually been able to perform the act of will, and cast the Ring into the Cracks.