So here is my perfectly valid circular argument: If the premise Baha’u’llah was a Messenger of God is true, then the conclusion God exists must be true.
But that wasn't your argument. Nor is it circular. You're no longer assuming that which you are trying to prove. Your argument above is formally valid. Nor is it an argument for the existence of a God. It's also not an argument that Baha'u'llah was a messenger of God whether a god exists or not.
It's also true that if Clement Moore, the author of It Was The Night Before Christmas, was a messenger of Santa, then Santa exists. It's not much of an argument for either Santa existing or Clements being his messenger. Yet this is what you call objective evidence for God. If Baha'u'llahs writings are objective evidence of a God, why is this poem not also objective evidence for Santa?
And I understand that you are offended by comparisons of a God with any other thing, but that's how reasoning often works. For you, God is real and these other things are not. But the people you are debating with do not hold that belief, and they are telling you why the God you propose exists has the same ontological status as anything else which it is claimed exists, but cannot be demonstrated. It doesn't matter which of them you choose to believe in or exalt.
No, it cannot ever be established that God exists without a Messenger of God (who is also a Manifestation of God) and who is the only evidence that God exists and God's mouthpiece and God's representative on earth.
I'm confused about where you see a god in all of this. I see a man and his words. Do the words reveal this God to you? If so, how? Is it because you consider them wise beyond what a human being could write without the help of a deity? If not, why are you calling it objective evidence for God?
No, I do not have to first prove that God exists before I can believe that anyone is God's Messenger. All I have to do is prove that the Messenger was sent by God and I have done so.
How do you propose to prove that a man was sent by a God that might not exist just by looking at the man or his words?
You only make yourself look foolish when you compare Messengers of God to magical pixies. The difference between the two is that there is no evidence that indicates that magical pixies exist whereas there is evidence that indicates that Messengers of God exist.
Perhaps foolish in your eyes because of your beliefs, but he doesn't look foolish to me. His argument is the same as mine using Santa, which is probably also foolish to you for the same reason, but once again, for those of us who don't share your beliefs, the comparison is apt. I suspect that some who do share your beliefs would agree that the comparison is apt. They would be the ones who tell you that they believe by faith.
Incidentally, he didn't compare messengers and pixies. He compared gods and pixies. And there isn't a lot of controversy about whether Baha'u'llah existed, but there is controversy about whether there is a god.