What is the meaning of "u'Fitachta ha'Sak me'Al Masnecha"?
Targum Yonasan: Bind sackcloth on your loins. Rashi - we must say so, for until now, Hashem did not command him to wear sackcloth, that now He would command him to remove it 1 . Also, he told him to remove his shoes, which is a sign of Aveilus. (Surely, He commanded also to wear sackcloth, which is a sign of Aveilus.) U'Fitachta is like "Mefutachos Pituchei Chosam" (Shemos 39:30); he should gird sackcloth tightly on his skin, so it will appear carved into his flesh. Me'Al Masnecha means above your loins.
Radak, Malbim: He was wearing sackcloth amidst mourning over the exile of the 10 tribes. Hashem commanded him to remove it and his shoes, and go naked and barefoot.
Perhaps Yeshayah was wearing sackcloth without any command! If so, the verse would have said so. (PF)
Was he truly naked?
Rashi, based on Targum Yonasan: No - he will wear torn and tattered garments.
Radak: Heaven forbid to say that Hashem would command the Navi to go naked and barefoot 1 ! Similarly, Hashem told Hoshe'a to marry a harlot, and Yechezkel to pass a razor over his head and beard, and to eat a cake of barley baked on cattle dung - He would not command a Navi to do so! Rather, Hashem commanded to do so in a prophetic vision, and he did so in the vision. Even though the time endured - here it was three years, and Hoshe'a married and fathered children - really, all was in a prophetic vision at once.
Malbim: Yes, but only for one day, or for a short time, in his house.
Is it difficult that Yeshayah would go barefoot? Hashem commanded Moshe to remove his shoes at the Sneh, and Kohanim serve barefoot in the Mikdash! (PF)
Why does it say "Na'alcha" (singular)?
