Camber - I have read the Ender series (OSC is actually one of my favorite authors...especially living, since most of my favorite authors are dead

) including Children of the Mind, so I'm familiar with the basic fictional concept which I knew Card borrowed. A question, though - if Christ is Jehovah, the Christ is the God of the Old Testament in its entirity, correct? Then the Father, like the Holy Spirit, was unknown to the world before the time of Christ, is that right?
I don't read Hebrew, but I understand that Elohim (who LDS identify as God the Father) is mentioned in the Genesis creation story. For example, we believe that when God says "we" and "us" in the creation story, he is talking to Jehovah, who created the earth under the authority of Elohim.
But you're basically right. Jesus is the God of the Old Testament. There have been very few times when God the Father has been seen or heard on earth. At the baptism of Jesus when John heard the Father's voice, for example, and at the stoning of Stephen, when he saw Jesus in heaven on the right hand of the Father.
Was he unknown before the NT? No, like the concept of a heavenly Mother, we believe that most prophets knew about the Father, Son, and Holy Spirit before the NT, but that the concepts weren't always taught to the masses.
There are few OT references, but I did a quick search and located 4 in the KJV:
Neh. 9: 20 Thou gavest also thy spirit to instruct them...
Neh. 9: 30 Yet many years didst thou aforbear them, and testifiedst against them by thy spirit in thy prophets...
Ezek. 36: 27 And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
Micah 3: 8 But truly I am full of power by the spirit of the Lord...
On the subject of heavenly Mother, I think the point is valid that what we do is pretty puny in relation to God. Personally, I think the commandments (i.e., thou shalt not steal, etc.) are more for our benefit rather than God's. So, when we are told not to take the name of God in vain, it probably (I think) because disrespecting God lowers us and separates us more from Him spiritually--not because He is ultra-sensitive and has an ego that cannot abide any disrespect. He wants us to progress and become more like Him, and has mapped out for us the pathway to get there.
So, although Perfection may have thought he was being really cute and cheeky in his comment about this topic, I don't think it's anything to joke around with, and I think the comment illustrates why we don't particularly need to know about heavenly Mother...we seem to be prone to using knowledge like this flippantly rather than treating it with reverence and awe.
It's also noteworthy though that while the prophets have not said much about heavenly Mother, you can find plenty of LDS writings on the topic. One that stands out in my mind was a near-death experience I saw written down back in 1992 in a collection of LDS essays. The writer believed that she had seen and interacted with her heavenly Mother, and the experience was important and meaningful to her (the writer).
Needless to say, there are people in Mormonism who would like more to be said about heavenly Mother, and they aren't just feminists. But for now the official word is that knowledge of heavenly Mother isn't essential for our salvation, so we don't need to know more about Her. Just as knowledge of other gods before heavenly Father, if there are any, isn't essential for our salvation either.
In response to the question of whether we would want to reverence or worship a "grandfather" god, the question, in my opinion, misses the point that God has given us everything we need for our salvation. If we embrace it and do our best with it, the possibilities beyond this life are limitless. There will be plenty of time then to satisfy our curiosity on such topics.