What is "She'ar Yashuv Benecha"?
Rashi: The small remnant that will return to me via you 1 ; they are like your children.
Radak: Yeshayah had two sons. One was named She'ar Yashuv, for a sign that the rest of Yehudah will return after Galus, but the 10 tribes will not return after the 70 years of Galus Bavel, and still did not return. They will return in the future. Also, Yehudah was not quick to be exiled like the 10 tribes were. The other son he called "Maher Shalal Chash Baz" (8:3), for a sign that the 10 tribes will be exiled before Yehudah.
Malbim: Yeshayah called his son She'ar Yashuv, to arouse that the rest of Yisrael will return to Hashem. Hashem commanded that he take him, to arouse him via a hint.
This is like Targum Yonasan, 'the rest of your Talmidim who did not sin, or repented from sin.'
Why did He say "El Ketzei Te'alas ha'Brechah"?
Rashi: You will find Achaz there. Te'alas is a ditch. Brechah is a pond made for fish. "Elyonah" - it is at the top of the slope of the mountain. There was another below.
Radak: Te'alas is an irrigation channel drawn from the Brechah. Brechah is built from rocks, for rainwater to gather in it.
Malbim: This is the pit and pond that Chizkiyah diverted due to Melech Ashur - "va'Asher Asah Es ha'Berechah v'Es ha'Te'alah va'Yavei Es ha'Mayim ha'Irah" (Melachim II, 20:20). Chachamim disagreed with him (Brachos 10b). Perhaps also Achaz went there to close the spring and the pond, lest the enemy find water to drink; Hashem commanded Yeshayah to stop him.
What is "Sedei Choves"?
Rashi citing Targum Yonasan: It is a field where launderers spread out garments to dry.
Rashi citing Sanhedrin 104a: Achaz was humbled in front of Yeshayah, and put on his head a perforated Kli that launderers use for sprinkling water on garments.
Radak: They used to launder garments there.
