JOEL - You are correct. Our God is the one and only God for us on this planet in this universe. That is all we really need to be concerned about. He is the one and only God that we must answer to. The Bible was written by prophets for all of us living on this planet in this universe. And so, when it talks about there being only one God, that doctrine has meaning for us who are living on this planet and in this universe. However, latter-day prophets have revealed to us that in the eternities, some people can and will become the "one and only" God for other people in some other universes.
Jesus said, "Be ye therefore perfect, even as your Father in Heaven is Perfect(Mat. 5:48). Since our Heavenly Father is a God, The only way we can become perfect like Him is to become a God also.
The concept of plurality of gods is mentioned in the Bible:
"For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,)(1 Corinthians 8:5)

I can not find the phrase "One man for one woman" anywhere in the King James Bible, but I think I understand what you are talking about.
In 1 Timothy 3:2 it says, "A bishop then must be blameless, the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach;"
At the time Paul was talking to the New Testament Christians it was not proper for a Bishop to have more than one wife. God can revoke or re-institute a practice or commandment as He sees fit. Such is the case with polygamy. There were prophets in Old Testament times who were told to have more than one wife. If God said it was O.K. then, why would it not be possible to be O.K. now?

The main problem with a lot of the questions that people have about the LDS Church today, is that they can not accept the fact that God himself is telling us these things through latter-day prophets, the same as He did in Biblical times.
There is a Mormon saying that goes, "Do the words of the prophet come from God? For those who believe in revelation, there is no question. For those who do not believe in revelation, there is no answer."

And now, as for the scriptures you refered to;
Galatians 1:6-9:
6. I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7. Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
8. But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
9. As we said before, so say I now again, If any [man] preach any other gospel unto you than that ye have received, let him be accursed.

2 Corinthians 11:3-4
3. But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ.
4. For if he that cometh preacheth another Jesus, whom we have not preached, or [if] ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with [him].

Paul's remarks in these two scripture references are specifically to the members of his church living there at that time. They had been taught the true Gospel and Paul was warning them to not accept anything else. Even if these scriptures are to be applied to us in these days, the LDS Church is not preaching "another gospel". It is preaching the same gospel that Paul was talking about, but by having latter-day prophets today, we have been able to learn more of it than was recorded in the Bible.

Salvation:
Salvation is redemption from the bondage of sin and death, through the Atonement of Jesus Christ. Jesus suffered for our sins and if we apply the principle of repentance and are obedient to God's commandments, we can be forgiven of our sins and be saved in heaven. Through the teachings of Jesus Christ and the revelations given to ancient and latter-day prophets, it is evident that there are different degrees or levels of salvation in the afterlife. Everyone will receive a reward in heaven that is in harmony with his conduct and obedience in life. Also, the Plan of Salvation provides for everyone to hear and respond to the gospel either in this life or in the spirit world, so that all who will, may be saved by obedience to the laws and ordinances of the gospel.