What you are saying is true, except that it's not useless because it informs us about what "God" means and also what morality actually is.  (In the same way, I wouldn't say that the sentence "Males have an XY chromosome" is useless, even though it is essentially a tautology.)  

Well stated. This is a very common word game that theists like to play.

To say that God is morally perfect is to say nothing more than 'God is God'. The question is then, why arbitrarily ground morality in God's nature? Indeed, there are many reasons not to!

I'm bringing up Old Testament ethics among a list of other things, including the resurrection.

One a venn diagram of things I believe are moral, and things associated with the Christian God, the amount of overlap is incredibly small.

Believe it or not, that is a red herring.  You aren't raising an ontological problem here, you are raising an epistemological problem with our ability to know what the objective morality is.

Your argument is like saying that because we each will have our own subjective interpretation of Shakespeare's intentions in writing King Lear, that therefore there is no objective intention that Shakespeare had in writing the play.  They are just completely separate issues; and simply because we might not epistemic access to God's objective morality does not mean that there is not an objective morality.

Moreover, it's possible to use greatest being theology to give an argument for the existence of objective morality completely apart from any attempt to define what that morality is.  By the nature of what it means to be God, God must be the greatest being that could logically exist.  If there could possibly be a being greater than what we think is God, then in reality what we think is God would not be the true God; but this greater being would.  So understood, God must have all great making properties, including the property of inherent righteousness and goodness.  It would be an objective fact that God would be necessarily good in nature.  Therefore those thoughts and actions that conflict with the nature of God are not good; if there were good actions, then they would have been a part of God's nature and thus would not have conflicted.  But given that they do conflict with God's nature they must necessarily be bad.  This would be true even if we had no idea what God's nature was, or which actions and thoughts conflicted with it.

I'm quite aware that they are different issues.

Agreed. 

Now, the reason why divine command theory isn't objective has been discussed above. I was raising a purely epistemological problem and I don't know how you got a different impression.

Any philosophy of morality necessarily involves a model of personhood upon which the behavior of people should be judged.  In many cases, the model is simply a Platonic ideal.  When you say that morality is determined by maximal well-being, or utilitarianism, or deontological theory or virtue ethics, you are simply positing your own model which happens to include whatever normative ethical base you wish for it to include.

However, God is purportedly the real instantiation of the ideal, a being whose moral nature is epistemically based not on what you wish to conjure arbitrarily but rather on special revelation.  So I didn't just choose God to be the standard of morality; rather, per the law of identity, he's necessarily the real model of personhood if he exists.  If I simply decided to use maximal well-being as my yardstick, I would, as you are doing, be conjuring my model arbitrarily; there's nothing intrinsic to well-being that equates it to moral rightness.  

This is the exact same red herring as before.  You said you understand the difference between epistemology and ontology, but your argument doesn't seem to reflect that. The chronological order of when I come to know things is not the same as the ontological causal order of their existence.

The moral argument goes like this:

1. If and only if God, then objective morality exists.

2. Objective morality exists

3. Therefore God exists.

Notice the first premise is a bi-conditional statement of equality.  The first premise states that the existence of God is identical to the existence of objective morality.  That is what we are arguing for with greatest being theology; that to know objective morality is to know God himself.  So even though I may have been aware of objective morality chronologically prior to knowing that this objective morality was in fact God; this in no way renders the argument invalid.