"Such thoughts as these were crowding through Isaiah's mind as he stood under the portico of the temple. Suddenly the gate and the inner veil of the temple seemed to be uplifted or withdrawn, and he was permitted to gaze within, upon the holy of holies, where even the prophet's feet might not enter. There rose up before him a vision of Jehovah sitting upon a throne high and lifted up, while the train of His glory filled the temple. On each side of the throne hovered the seraphim, their faces veiled in adoration, as they ministered before their Maker and united in the solemn invocation, “Holy, holy, holy, is the Lord of hosts: the whole earth is full of His glory,” until post and pillar and cedar gate seemed shaken with the sound, and the house was filled with their tribute of praise. Isaiah 6:3 . PK 307.1"
"In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple."
The EGW passage is a direct retelling and amplification of Isaiah 6:1, describing Isaiah's vision of God on His throne. It even quotes Isaiah 6:3.
"And one cried unto another, and said, Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory."
The EGW passage quotes Isaiah 6:3 directly and places it within the context of Isaiah's vision.
"Above it stood the seraphims: each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly."
The EGW passage directly describes the seraphim as described in Isaiah 6:2, including the detail about their wings covering their faces and feet.
"the full light of the divine presence within the inner sanctuary, he realized that if left to his own imperfection and inefficiency, he would be utterly unable to accomplish the mission to which he had been called. But a seraph was sent to relieve him of his distress and to fit him for his great mission. A living coal from the altar was laid upon his lips, with the words, “Lo, this hath touched thy lips; and thine iniquity is taken away, and thy sin purged.” Then the voice of God was heard saying, “Whom shall I send, and who will go for Us?” and Isaiah responded, “Here am I; send me.” Verses 7, 8 . PK 307.2"
"Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? Then said I, Here am I; send me."
EGW quotes Isaiah 6:8 directly and uses it as a central point in her narrative.
"Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the Lord of hosts."
EGW directly references and expands upon Isaiah's feeling of inadequacy before being cleansed.
"Then flew one of the seraphims unto me, having a live coal in his hand, which he had taken with the tongs from off the altar: And he laid it upon my mouth, and said, Lo, this hath touched thy lips; and thine iniquity is taken away, and thy sin purged."
The EGW passage directly quotes Isaiah 6:6-7 regarding the seraph touching Isaiah's lips with a coal and the subsequent cleansing. This is a clear and direct allusion.