Prince_James, you state in the OP that God is either necessarily existent or necessarily non-existent, but you don't say why.
Why do you dismiss the "not-necessarily existent or not-necessarily non-existent" options?
Further, you say that IF God's necessity (one way or the other) is provable by reason alone then agnosticism is a flawed position - yet noone has ever managed to prove God's necessity by reason - and you have not given any argument that suggests otherwise.
The Ontological argument is all well and good, if that is what you are basing this discussion on - until you realise that all they are describing is existence itself, not God. And that existence is necessarily existent. (NB: I think one could go further and equate existence to the Universe.)
Nothing that can be said about God through the Ontological argument can not be attributed to existence.
But since we already have a word for existence... which is... er... "existence", we have no use for the word "God".
If you think differently, please explain what the Ontological Argument says about God, and we'll compare it to "Existence".
Or if you are not basing this on the OA, what exactly are you arguing? That IF God is provable by reason THEN the agnostic position would be flawed? Well, duh, yeah. IF.
But if you're saying that God IS provable by reason alone - please demonstrate, and prove.