First of all, I'd propose that we could talk about something else rather than God, since we are having a discussion about believing in a thing, not necessarily religious faith. Bertrand Russel used the example of a teapot in orbit near Mars, and that will suffice. I think that will make it easier to have a discussion about believing things and why we believe them without introducing religious faith into the mix. So, I'll use the teapot.
Secondly, I disagree with your definitions. I find the following diagram much closer to how I would phrase things.
With this, I would be somewhere in the bottom right section. I lack belief in God, and I'm very confident of my position, but I wouldn't say 100% confident.
As such, it seems to me that agnostic atheism is a fairly strong position. It doesn't accept anything for which there is little to no reliable evidence, yet it does not rule it out. It remains open to the possibility, but it would require some pretty strong evidence to be convinced.
Going back to the earlier teapot analogy, I'm sure you aren't going around saying, "I absolutely INSIST that there is no teapot near Mars! I am absolutely convinced that such a thing is utterly impossible!!!" Such a position is analogous to what would be held by a gnostic atheist. The far more rational thing to say would be, "I don't believe that there is a teapot near Mars, since there's no explanation that I've been told of that holds up to scrutiny. However, I can't deny that there could be some explanation that I'm unaware of, and so I can't claim for certain that the teapot doesn't exist." That is the position of the agnostic atheist.
To continue the analogy, the gnostic theist would claim that there can't be any doubt at all that the teapot is there, it absolutely MUST be there, and anyone who can't see that is a fool. This is clearly a ridiculous position. And the agnostic theist would claim that even though there is no indisputable proof, it is clearly more likely that the teapot is there than not, so we should believe that the teapot is there, even without sufficient evidence.
However, assuming that you disagree with my criticisms of your definitions, I still don't see how atheism is the weakest in your original argument. If you are claiming that it is weakest because it is making an argument of certain knowledge, then theism must be just as weak.