Now as for your argument, I think two of its premisses are false. First, it seems to me that (2) is false, both on philosophical and on theological grounds. Philosophically, I’m persuaded by arguments such as have been offered by Harry Frankfurt that free choice does not entail the ability to do otherwise. Imagine that a mad scientist has secretly wired your brain with electrodes so that he can control your choices. Suppose that in the last Presidential election, he wanted you to vote for Obama and had determined that if you were going to vote for McCain he would activate the electrodes and make you cast your vote for Obama. Now as it turns out, you also wanted to vote for Obama, and so when you went into the polling booth you marked your ballot for Obama, and therefore the scientist never activated the electrodes. I think it’s clear that you freely voted for Obama, even though it was not possible for you to do otherwise. What this thought experiment suggests is that the essence of free choice is the absence of causal constraint with respect to your choices; it is up to you alone how you choose.
Now in the case of God, if God is essentially good, then there is no possible world in which He does evil. But does that imply that God does not freely do the good? Not if Frankfurt’s analysis is right. For God acts in the complete absence of any causal constraint whatsoever upon Him. It is up to Him and Him alone how He acts. He therefore acts freely in doing the good. That God is acting freely is evident in the fact that His will is not inclined necessarily toward any particular finite good; He chooses to do whatever He wants.