Why did He tell the one who wears linen?
Malbim: Burning the city and the Mikdash were amidst mercy, like I explained. Therefore He commanded the advocate to do this.
Malbim (the Nimshal): He commanded Yirmeyah to prophesize about burning the city and the Mikdash, for it is for the good of Yisrael and their salvation.
Why does it say "va'Yomer" twice?
Rashi, based on Yuma 77a and Eichah Rabah 1:41: This is always for a Drashah. Hashem told Gavriel, and he requested from the Keruv to give to him, in order that the coals will cool off and the decree will be lightened - "va'Yisa va'Yiten El Chofnei Levush ha'Badim" (7).
Where is "Beinos la'Galgal"?
Rashi: It is between the Ofanim. Radak - the prefix in la'Galgal is in place of Hei ha'Yedi'ah (to identify). Also la'Keruv is like ha'Keruv, mi'Beinos la'Keruvim is like ha'Keruvim 1 . It says la'Galgal (singular) regarding the Klal; it is as if it says la'Galgalim. Also "la'Ofanim Lahem Korei ha'Galgal b'Aznai" (13) and la'Keruv in our verse refer to the plural in the singular, i.e. the Klal.
Malbim: The Galgal is not the Ofan itself. Rather, it is what makes the Ofan roll. It is the Chayos' feet, which are like the soles of a calf.
Radak: Also "la'Melech Nish'an" (Melachim II, 7:2) is like ha'Melech; "Ad la'Shamayim Higi'a" (Divrei ha'Yamim II, 28:9) is like ha'Shamayim. Shir la'Ma'alos (Tehilim 121:1) is like ha'Ma'alos.
What is the meaning of "El Tachas la'Keruv"?
Radak: This is like Beinos la'Keruv. It says Tachas to hint that the Galgal is a lower level than the Keruv.
Malbim: This is the place of the soles of their feet.
Are the coals between the Keruvim?
Radak: This is like mi'Tachas la'Keruvim. Malbim - he could not take fire from the Ofanim and Ma'arachah itself, for the Ma'arachah did not obligate burning, rather, eradication of the nation. The Keruvim moved the Ma'arachah unlike its nature, and overrode it, based on the command of the Exalted who sits on the throne, to burn the city and save a remnant of Yisrael. 1
Radak: This is like it says in the first [vision of the] Merkavah "Mis'haleches Bein ha'Chayos" (1:13); we explained this there.
Did he throw them on the city immediately?
Rashi, From Eichah Rabah 1:41: No. Even though he received them [now], he did not throw them until six years later. They dimmed in his hands all six years. We must say so, for this Nevu'ah was in the sixth year [from Galus Yechanyah] (8:1), and the city was destroyed in the 11th year.
What came to his eyes?
Radak: He came between the Galgalim, like Hashem commanded, and I saw him come. Yechezkel did not finish telling what he did; he interrupted to discuss the Keruvim.