Why does the Navi say when this was?
Rashi (1:1, from Mechilta, Shirah 7): This truly begins Sefer Yeshayah. What came before in the Sefer is about Yehudah and Yerushalayim, just like "Masa Bavel", "Masa Mo'av" (13:1, 15:1) are about Bavel and Mo'av. The Navi is not [always] in chronological order. We must say so, for on the day of the great noise, Uziyah became a Metzora, and Hashem said "whom will I send?. and I said 'here I am, send me'" (6:8). That was the start of his Shelichus; the Nevu'ah here was said afterwards in the days of Chizkiyah, after the 10 tribes were exiled. Hagahah in Rashi (here) attributed to Abarvenel 1 - [another] proof is "v'Nosrah Vas Tziyon k'Sukah" (1:8), which was after Sancheriv exiled the 10 tribes (during Chizkiyah's reign, long after Uziyah's death mentioned here)!
Surely Rashi did not write this, for it discusses the Perakim of the Sefer (PF). Tanach (other than Tehilim and Eichah) was divided into Perakim long after Rashi died, initially via Goyim. The Perakim became accepted in Yisrael to facilitate refuting heretics in mandatory debates. (Ohr Yisrael (Monsey) 5758 12, p.144)
Why does it say "in the year of Uziyah's death", and not 'at the beginning of Yosam's reign'?
Rashi, from Targum Yonasan: It refers to when Uziyah became a Metzora (and not when he truly died). Radak - this is when he entered the Heichal to burn Ketores. Tzara'as came on his forehead.
Radak: It was when he truly died. It is attributed to his death, for then the nation did evil.
How could he see Hashem? "Ki Lo Yir'ani ha'Adam va'Chai" (Shemos 33:20)!
Radak citing Targum Yonasan: He saw Hashem's honor. Also "va'Yered Hashem Al Har Sinai"(Shemos 19:20), Targum Onkelos is 'v'Isgali Yekara 1 da'Shem.' Yechezkel said "v'Al Demus ha'Kisei Demus k'Mar'e Adam mil'Ma'alah" (Yechezkel 1:26, refers to His honor). Here, "Ram v'Nisa" refers to the throne. It was all a prophetic vision.
Radak citing Yevamos 49b: This refers to a glass that distorts, and this refers to a clear glass 2 . Rashi (49b) - all (other) Nevi'im saw Hashem through a glass that distorts; only Moshe realized that he did not really see Hashem.
Malbim: Here does not discuss physical vision, rather, intellectual. One cannot understand Hashem Himself, only His actions.
Our text of Onkelos is 'v'Isgali Hashem'! However, Targum Yonasan of "va'Yered Hashem b'Amud Anan" (Bamidbar 12:5) is 'v'Isgali Yekara da'Shem.' (PF)
Malbim: One cannot understand Hashem Himself. One can attain a semblance of His honor via His actions, via the connection of outcomes to the First cause. This is like one who senses the sun's light - he does not see the sun itself, only light that comes from something that received its light.
Why does it say that He was sitting on a throne?
Malbim: Hashem's conduct is seen in two ways. (a) Natural conduct fixed from the beginning that does not change. The Tzeva ha'Shamayim, via their movement, they mix and compose the elements, and they are prepared to receive different forms. (b) Conduct via miraculous Hashgachah, which overrides nature when needed. This is via Tzeva ha'Mal'achim; it is higher conduct. It applies only for a great need. Here the Navi describes the former. Sitting represents something fixed. The Kisei is Tzeva ha'Shamayim - "ha'Shamayim Kis'i veha'Aretz Hadom Raglai" (66:1), "Hashem ba'Shamayim Hechin Kis'o" (Tehilim 103:19).
Why does it say "Ram v'Nisa"?
Malbim: Since the verse said that Hashem sits on a throne, it comes to distance from Him physicality, lest the metaphor be understood simply. Hashem is elevated above these descriptions, which speak in a way that we can understand.
Radak (3, citing the Kuzari): He is elevated from any effect of the Tum'ah of the nation amidst which He made His honor dwell.
What are "Shulav"?
Rashi: It is like "Shulei ha'Me'il" (Shemos 28:34) - its bottom. I saw Him sitting on a throne in Shamayim, and his feet were in the Heichal, Hadom Margelosav, in the Beis ha'Mikdash, to judge Uziyah, who came to take the crown of Kehunah.
Radak #1, based on Targum Yonasan: It is the bottom of His honor.
Radak #2: It is the bottom of His Kisei. Malbim - it is the end of natural conduct.
What is the meaning of "Mele'im Es ha'Heichal"?
Radak #1: This is like Memal'im (they fill). This is like "u'Chevod Hashem Malei Es ha'Mishkan" (Shemos 40:34). "Ha'Heichal" is the Heichal of the Beis ha'Mikdash.
Radak #2: "Ha'Heichal" is Shamayim - "Hashem b'Heichal Kodsho Hashem ba'Shamayim Kis'o" (Tehilim 11:4).
Malbim: The end of the authority of the Kisei, i.e. the development of conduct of Shamayim, descends to the Heichal, to spread from there to the lower world - "veha'Aretz Hadom Raglai" (66:1). Also conduct of the land is not general, rather, with personal Hashgachah; nothing is empty from His Hashgachah.
Why did Yechezkel describe his vision in much greater detail than Yeshayah?
Radak citing Chagigah 13b: Yechezkel was like a villager who saw the king. Yeshayah was like a city resident who saw the king. I.e. Yeshayah (or his generation) was at a higher level of perfection than Yechezkel (or his generation), so what he saw was not such a Chidush to him.
