JOEL - God was not lying. He was telling the people only what they would be able to understand and only what they needed to know. You state that "God is eternal by definition". Yes He is eternal. He has always existed as either an intelligence, or as a spirit being, or as a resurected immortal being. Before He was born spiritually of His heavenly parents, He existed eternally as an intelligence(D&C 93:29), destined to be the God of this universe which did not yet exist. His intelligence combined with His spirit and, being the firstborn spirit, was chosen and ordained to create this universe and be the one and only Savior and Redeemer of the human race (Abr. 3:21-23).
So in this way He is eternal. He was never "formed". The nature of His being may have change throughout the past present and future eternity, but even as an intelligence He was destined to be our God and therefore He has been and always will be our God. We follow a similar path in that we have always existed as either inteligences, spirits, or mortal beings, and may someday in the eternities become as God ourselves.
Also when this scripture is not read out of context you get a slightly different understanding of it.
In verse 1 of Isaiah 43 the Lord identifies who he is talking to:
"BUT now thus saith the LORD that created thee, O Jacob, and he that formed thee, O Israel, Fear not: for I have redeemed thee, I have called [thee] by thy name; thou [art] mine."
Here God is not talking to or about people of other universes that have or will exist. He is talking to those whom He created. He is ours, we are His. His authority is over us and no one else.
Then if you look at verse 11 God says, "I, [even] I, [am] the LORD; and beside me [there is] no saviour." Each universe that has existed or will exist will have its own Savior. God is saying that He is the only God and only savior that we will need. Only He will save us and redeem us from death. Everything God said in the Bible is for us and only us, existing at this time in this universe.
Therefore when He says, "there was no God formed, neither shall there be after me" Christ is declaring that He is the only one the Father exalted to authority over His fellow brothers and sisters, not that His brothers and sisters cannot become like Him and His Father.
These things are hard for us to understand now with our finite mortal minds. Of course much of what I have explained here we only know through latter-day revelation. But not I nor anyone in our church understands this perfectly. We are still too immature in our path on the way to eternal life to be able to comprehend it all now. We are also not all that certain that every scripture in the Bible is saying exactly what it was meant to say. Anyone who has studied the Bible is aware of a few of the contradictions and errors that exist in it. (See http://www.mormonhaven.com/bible.htm)
My explanation will probably not satisfy you, but this is how we interpret the scripture based on what we know to be true.