Was Psalms 45:6 Mistranslated? I'm having difficulty with this passage.

On Psalms 45:6:


  Your throne, O God, is forever and ever; A scepter of uprightness is the scepter of Your kingdom. You have loved righteousness and hated wickedness; Therefore God, Your God, has anointed 


I don't know how the author of Psalms could have referred to a King as God in this verse. Wouldn't a better translation for "elohim" here simply be "judge"?

Next the author of Hebrews quotes this verse and applies it to the Son. Hebrews 1:8-9


  But about the Son He says: “Your throne, O God, will last forever and ever, and righteousness will be the scepter of Your kingdom. You have loved righteousness and hated wickedness; therefore God, Your God, has placed You above Your companions by anointing You with the oil of joy..


So why did the author of Hebrews take this arguably mistranslated passage and apply it to the Son?
According to Bible scholar B. F. Westcott, this is likely a Greek to English mistranslation, not a Hebrew to Greek mistranslation. Based on the context of the verse, he believes a better rendering would be: &quot;God is Thy throne&quot; or &quot;Thy throne is God&quot;.
He says (bold mine, italics original):

The LXX. admits of two renderings: [ho the·osʹ] can be taken as a vocative in both cases (Thy throne, O God, . . . therefore, O God, Thy God . . . ) or it can be taken as the subject (or the predicate) in the first case (God is Thy throne, or Thy throne is God . . . ), and in apposition to [ho the·osʹ sou] in the second case (Therefore God, even Thy God . . . ). . . . It is scarcely possible that [’Elo·himʹ] in the original can be addressed to the king. The presumption therefore is against the belief that [ho the·osʹ] is a vocative in the LXX. Thus on the whole it seems best to adopt in the first clause the rendering: God is Thy throne (or, Thy throne is God), that is ‘Thy kingdom is founded upon God, the immovable Rock.’
—The Epistle to the Hebrews (London, 1889), pp. 25, 26.

There is at least one Bible translation which uses this rendering: the New World Translation.

God is your throne forever and ever;
The scepter of your kingdom is a scepter of uprightness.
You loved righteousness, and you hated wickedness.
That is why God, your God, has anointed you with the oil of exultation more than your companions.
—Psalm 45:6, 7
But about the Son, he says: “God is your throne forever and ever, and the scepter of your Kingdom is the scepter of uprightness. You loved righteousness, and you hated lawlessness. That is why God, your God, anointed you with the oil of exultation more than your companions.”
—Hebrews 1:8, 9

Jehovah's Witnesses have also published an article discussing this translation of Hebrews 1:8.

RS reads: “Of the Son he says, ‘Thy throne, O God, is for ever and ever.’” (KJ, NE, TEV, Dy, JB, NAB have similar renderings.) However, NW reads: “But with reference to the Son: ‘God is your throne forever and ever.’” (AT, Mo, TC, By convey the same idea.)
Which rendering is harmonious with the context? The preceding verses say that God is speaking, not that he is being addressed; and the following verse uses the expression “God, thy God,” showing that the one addressed is not the Most High God but is a worshiper of that God. Hebrews 1:8 quotes from Psalm 45:6, which originally was addressed to a human king of Israel. Obviously, the Bible writer of this psalm did not think that this human king was Almighty God. Rather, Psalm 45:6, in RS, reads “Your divine throne.” (NE says, “Your throne is like God’s throne.” JP [verse 7]: “Thy throne given of God.”) Solomon, who was possibly the king originally addressed in Psalm 45, was said to sit “upon Jehovah’s throne.” (1 Chron. 29:23, NW) In harmony with the fact that God is the “throne,” or Source and Upholder of Christ’s kingship, Daniel 7:13, 14 and Luke 1:32 show that God confers such authority on him.