Pete
The confusion is in part due to the fact that the translators have to discern whether the expression "And his/His name will be called..." refers to Yahweh or the king. They are split and so in some cases you see the "His" as capitalized referring to God and in others not when interpreted as reference to the king. Also this section is separated by a semi-colon or not. It is then either Yahweh who "shoulders the government" and "sustains it with justice and righteousness" or this representative king. The least complicated understading is that these expressions in question refer to Yahweh, given the reference to the "throne"(kingship) as a separate entity being blessed in verse 6.
I think the least complicated understanding is that these expressions refer (just as it reads) to both, Yahweh, The King
Forgive me if I except Matthew's translation over yours.
Mat 1:22
Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, :23 Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.