Before you call anyone incompetent, you should learn some Greek grammar!
In John 20:28, Thomas addresses Jesus Christ, "Ὁ Κύριός μου καὶ ὁ Θεός μου", this is in the nominative, as in Hebrews 1:8. Yet all translate this as "My Lord and my God".
From the time of Homer, and in attic, classical and koine Greek, there are MANY instances where the nominative is used for the vocative, in addresses!
The New Testament by the Unitarian, Dr George Noyes, reads: “but of the Son: ‘Thy throne, O God, is for ever and ever”
Another Unitarian, Dr George Winer, also admits that in Hebrews 1:8, the vocative is to be understood.
“The nominative (with the article) is sometimes used in an address, particularly in calling or commanding, thus taking the place of the vocative…H. i.8” (A Treatise on the Grammar of New Testament Greek, p.227)
The Hebrew scholar, Aquila, who published a Greek Version of the Old Testament, in the middle of the 2nd century A.D., translates the Hebrew, by the Greek, “ο θρονος σου θεε”, which is undoubtedly the vocative, “Your throne, O God”. (Fredrick Field, Origen Hexapla, vol. II, pp. 162-163)
As is the reading of another Jewish Bible, “Thy throne, O God” (Dr A Benisch; Jewish School and Family Bible, Vol.IV).
The Jehovah’s Witnesses, in both their Kingdom Interlinear New Testaments, 1969 and 1985, read in the English in the right-hand column, “God is your throne forever”. However, in the literal English translation under the Greek text, it reads: “the throne of you the God”. In the other Greek Interlinear that the JW’s publish, The Emphatic Diaglott, it is even more interesting. In verse 8, the literal English translation under the Greek text, it reads: “concerning but the Son; the throne of thee the God for the age [of the age]”. And in the English version in the right-hand column, “But to the Son, Thy Throne, O God is for the age”. And, for verse 9, “therefore thy God anointed thee, O God” (right-hand column). Both verses in this translation has the vocative.