They were not forgotten. But they were far beyond aid, and no thought could yet bring any help to Samwise Hamfast’s son; he was utterly alone.

There he halted and sat down. For the moment he could drive himself no further. He felt that if once he went beyond the crown of the pass and took one step veritably down into the land of Mordor, that step would be irrevocable.

As it drew near the great furnaces where, in the deeps of time, it had been shaped and forged, the Ring’s power grew, and it became more fell, untameable save by some mighty will. As Sam stood there, even though the Ring was not on him but hanging by its chain about his neck, he felt himself enlarged, as if he were robed in a huge distorted shadow of himself, a vast and ominous threat halted upon the walls of Mordor. He felt that he had from now on only two choices: to forbear the Ring, though it would torment him; or to claim it, and challenge the Power that sat in its dark hold beyond the valley of shadows. Already the Ring tempted him, gnawing at his will and reason. Wild fantasies arose in his mind; and he saw Samwise the Strong, Hero of the Age, striding with a flaming sword across the darkened land, and armies flocking to his call as he marched to the overthrow of Barad-dûr. And then all the clouds rolled away, and the white sun shone, and at his command the vale of Gorgoroth became a garden of flowers and trees and brought forth fruit. He had only to put on the Ring and claim it for his own, and all this could be.

In that hour of trial it was the love of his master that helped most to hold him firm; but also deep down in him lived still unconquered his plain hobbit-sense: he knew in the core of his heart that he was not large enough to bear such a burden, even if such visions were not a mere cheat to betray him. The one small garden of a free gardener was all his need and due, not a garden swollen to a realm; his own hands to use, not the hands of others to command.

`Yes! The Elf-warrior is loose!’ he cried. ‘I’m coming. Just you show me the way up, or I’ll skin you!’

‘Gilthoniel, A Elbereth!’ Sam cried. For, why he did not know, his thought sprang back suddenly to the Elves in the Shire, and the song that drove away the Black Rider in the trees.

‘Aiya elenion ancalima!’ cried Frodo once again behind him.

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I like that Sam uses Sindarin and Frodo uses Quenya. Is there any significance to that? And how does Sam always know what to say or sing? Does anyone find it contrived that Sam suddenly recalls words spoken in Sindarin in the Shire half a year ago? Or can suddenly put words to one of Bilbo's songs?

Is there any significance in Tolkien's referring to Sam here as "Samwise Hamfast's son"?

At first he could see nothing. In his great need he drew out once more the phial of Galadriel, but it was pale and cold in his trembling hand and threw no light into that stifling dark. He was come to the heart of the realm of Sauron and the forges of his ancient might, greatest in Middle-earth; all other powers were here subdued.

[Eowyn] ‘I stand in Minas Anor, the Tower of the Sun,’ she said; ‘and behold! the Shadow has departed! I will be a shieldmaiden no longer, nor vie with the great Riders, nor take joy only in the songs of slaying. I will be a healer, and love all things that grow and are not barren.’ And again she looked at Faramir. ‘No longer do I desire to be a queen,’ she said.

They were like great figures seated upon thrones. Each had three joined bodies, and three heads facing outward, and inward, and across the gateway. The heads had vulture-faces, and on their great knees were laid clawlike hands. They seemed to be carved out of huge blocks of stone, immovable, and yet they were aware: some dreadful spirit of evil vigilance abode in them. They knew an enemy. Visible or invisible none could pass unheeded. They would forbid his entry, or his escape.

Hardening his will Sam thrust forward once again, and halted with a jerk, staggering as if from a blow upon his breast and head. Then greatly daring, because he could think of nothing else to do, answering a sudden thought that came to him, he drew slowly out the phial of Galadriel and held it up. Its white light quickened swiftly, and the shadows under the dark arch fled. The monstrous Watchers sat there cold and still, revealed in all their hideous shape. For a moment Sam caught a glitter in the black stones of their eyes, the very malice of which made him quail; but slowly he felt their will waver and crumble into fear.

Snaga: "You heard the bell. He’s got past the Watchers, and that’s tark’s [Gondorian] work."

They seem much too evil on the surface to be anything Gondor made, though it's possible they were Gondorian structures resculpted by Sauron & Co.

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I understand why this seems to imply Gondor made the Watchers. My explanation is that Gondor made the fortress, so maybe a Gondorian could foil the fortress's spells against them. Or maybe the magic tradition of Gondorians, which enabled Isildur to put a curse of undying on the King of the Mountains, is legendary enough that the orcs still think they're magical.

When thirteen had fallen the rest fled shrieking, leaving the defenders unharmed, except for Sam who had a scratch along the scalp. A quick duck had saved him; and he had felled his orc: a sturdy thrust with his Barrow-blade. A fire was smouldering in his brown eyes that would have made Ted Sandyman step backwards, if he had seen it.

Grief at last wholly overcame them, and they wept long: some standing and silent, some cast upon the ground.

The limit of the Willing Suspension of Disbelief for a given element is directly proportional to its awesomeness.

Stated another way, all but the most pedantic of viewers will forgive liberties with reality as long as the result is wicked sweet or awesome. This applies to the audience in general; there will naturally be a different threshold for each individual. Also known in some circles as a "rad herring", in which something doesn't make sense within the guidelines of the story's reality, but it's too cool not to include it.
The Rule of Cool - article on TVtropes.

Common gripes I see about stories include that it looks too much like success is achieved by unlikely co-incidences, or that someone acts out of character because it's convenient for the storyteller, or that something else happens that is not 'believable'.

the path of Sam's Ring-temptation here is via his desire to protect Frodo; to bear the burden for him. As you say, the thought of having power over others had almost no purchase on Sam. But telling himself that he would be protecting Frodo in bearing the Ring touches that great heart of his.