First, we should note that the Dead Sea Scrolls version of Isaiah confirms the Masoretic reading, although this still does not put us earlier than the LXX. However, this fact does narrow the distance between the LXX and the precursor of the MT by as much as a millennium. 

But more important, Jewish translators do not accept the typical Christian translation of this verse. One [Jewish rendering](https://www.chabad.org/library/bible_cdo/aid/15940/jewish/Chapter-9.htm) of the text indicates that, rather than the child being given such titles as "mighty God" and "everlasting Father," these titles are ascribed to God, who names him simply "Prince of Peace." 

> For a child has been born to us, a son given to us, and the authority
> is upon his shoulder, and the wondrous adviser, the mighty God, the
> everlasting Father, called his name, “the prince of peace.”

Another Jewish version handles the translation this way:

> For a child has been born to us, A son has been given us. And
> authority has settled on his shoulders. He has been named “The Mighty God is planning grace; the Eternal Father, a peaceable ruler."[Pele-yo῾eż-el-gibbor-avi-῾ad-sar-shalom.] (JPS)

(BTW this verse is Isaiah 9:5 in Jewish Bibles and some Catholic versions.)

**Conclusion**: Regarding the OP's main question, the LXX is not the original, because the original was written in Hebrew. Since the Masoretic and the DSS versions agree, they are more likely to be closer to the original. However, the Christian translation of the Hebrew tends to support a Christian understanding of the Messiah as a divine being, while the Jewish translations tend to support the idea that Messiah is a human being.