Folks, I'm disappointed. All we have had on this thread so far are the same tired old arguments against a literal, earthly reign of Christ held by postmil, amil, and preterist folk. However, the OP was about a specific exegesis.
Again: if Is. 9:6 is a prophecy of the incarnation which all Bible-believers (whatever their eschatology) interpret literally, why is v. 7. which is about the throne of David, not to be interpreted literally? I say that an interpretive key is the fact that every single mention--every single one--of the throne of David up until this passage was a literal earthly throne.
Folks, I've done the hard work, hours of study, but I'll share it with you. Here are the verses:
Solomon on David's throne: 1 Kings 1:13, 1:37, 1:47, 2:12, 2:24, 3:6, 5:5, 8:20, 8:25, 1 Chron. 29:23, 2 Chron. 6:10, 7:18, 2Ch 6:16.
Otherwise, before Isaiah 9 but not messianic:
2 Sam. 3:10--"To translate the kingdom from the house of Saul, and to set up the throne of David over Israel and over Judah, from Dan even to Beersheba.
Ps. 122:5--"For there are set thrones of judgment, the thrones of the house of David."
In Jeremiah, all literal earthly thrones: 13:13, 17:25, 22:2-4, 22:30, 29:16, 36:30.
Then there are the Messianic prophecies:
1Ki 2:33 Their blood shall therefore return upon the head of Joab, and upon the head of his seed for ever: but upon David, and upon his seed, and upon his house, and upon his throne, shall there be peace for ever from the LORD.
1Ki 2:45 And king Solomon [shall be] blessed, and the throne of David shall be established before the LORD for ever.
1Ki 9:4 And if thou wilt walk before me, as David thy father walked, in integrity of heart, and in uprightness, to do according to all that I have commanded thee, and wilt keep my statutes and my judgments: 5 Then I will establish the throne of thy kingdom upon Israel for ever, as I promised to David thy father, saying, There shall not fail thee a man upon the throne of Israel.
Ps 132:11 The LORD hath sworn [in] truth unto David; he will not turn from it; Of the fruit of thy body will I set upon thy throne. 12 If thy children will keep my covenant and my testimony that I shall teach them, their children shall also sit upon thy throne for evermore.
Isa 9: 6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. 7 Of the increase of [his] government and peace [there shall be] no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
Isa 16:5 And in mercy shall the throne be established: and he shall sit upon it in truth in the tabernacle of David, judging, and seeking judgment, and hasting righteousness.
Jer 33:19 And the word of the LORD came unto Jeremiah, saying, 20 Thus saith the LORD; If ye can break my covenant of the day, and my covenant of the night, and that there should not be day and night in their season; 21 Then may also my covenant be broken with David my servant, that he should not have a son to reign upon his throne; and with the Levites the priests, my ministers. 22 As the host of heaven cannot be numbered, neither the sand of the sea measured: so will I multiply the seed of David my servant, and the Levites that minister unto me.
Lu 1:32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: 33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.