"2 Peter 1:20 'knowing this first, that no prophecy of Scripture is of any private interpretation'"
This one you have to understand the background. Peter, like Paul, was writing to the members of the Church who were reverting back to their old ways. In the next chapter Peter warns them to beware of false prophets. So in that light he was telling them not to interpret the scripture through the eyes of their old religion. He didn't want them to come with ideas without his council.
In all practicality the Bible cannot be read without private interpretation. However, there are avenues that one can go down for verification of what it means. In all things ask God.
"If God doesn't say it, I'm not going to believe it just because He doesn't explicitly rule it out in one passage."
I am not asking you to believe what I am saying, I am just explaining what we, members of the Church of Jesus Christ of Latter-day Saints, believe. I am not by any stretch of the imagination, a spokesperson for the Church. I am explaining based on what I know.
The fact of the matter is God actually did tell us about the 3 kingdoms. You just don't know about the sources of this information, I also suspect you are not interested in reading those those scriptures. That's okay. It is your life. However, should you change your mind and want to know what else God has revealed then you can ask BritishMormon or myself.
No, the Bible is not all metaphor. Most of it isn't. Jesus taught in parables and those are essentially the same thing as a metaphor.
"2 Thessalonians 1:8-9 'In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:'"
Unless a person is righteous, is moved upon by the Holy Ghost or has the priesthood authority of God they will not be able to stand in God's or Jesus' presence. They will die. It is not out of vengeance, it is a matter of worthiness.
"Revelation 20:15 'And whosoever was not found written in the book of life was cast into the lake of fire.'"
Much of the book of Revelation is metaphor or symbolism. There is a book of life but the lake of fire is a metaphor, not an actual place. It just makes no sense at all that God would have so many children just to throw a majority of them into a lake of fire for eternity. That is not a God of love. That is not a God who chose as his title, "Father".