1)
Who returned him, and from where?
1.
Malbim: The angel, from Chatzer ha'Penimi.
2)
What is "[Sha'ar] ha'Mikdash ha'Chitzon"?
1.
Radak: It is [the gate of] the Heichal.
2.
Malbim: It is the eastern gate of Chatzer ha'Chitzonah, which faces east; he initially entered via it.
3)
How did he return via the gate if it is closed?
1.
Radak: Now that he returned me, I found it closed. It was open when Hashem's honor entered the Bayis, like it says above (43:2).
2.
Malbim: He found it closed and could not leave through it.