This is rather circular, don't you think?
The other definitions cannot be considered irrelevant if the argument is about the definitions.
All or nearly all concepts that derive from "deity" are circular and arbitrary, because they inherit those properties from that of deity.
And that means that everyone is entitled to simply pick and choose whichever definitions they like best. Some care should be taken to avoid switching concepts mid-argument, but that is all.
The basic dilemma is that you don't accept concepts of atheism that include lack of capability for conceiving a god. But one can just as legitimaly (or IMO
considerably more legitimaly) state that since atheism is the negative concept and theism is the positive concept, it is theism that needs justification.
Atheism
is the default, and basically meaningless in and of itself.
I have multiple reasons for considered the "lack of belief" definition a rather poor one.
And you are free to keep them, or even to attempt to convince us that we should adopt at least some of them.
I don't expect that we will, but it is your call.
But, don't you find this stance a bit hypocritical? Are you not dismissing inconvenient definitions yourself, ones that would exclude babies as possible atheists?
It is not at all hypocritical to acknowledge that some concepts are arbitrary and therefore a judgment call must be made.
Mestemia and me, among others, simply find it so much more logical to use definitions that include rocks and babies among the ranks of atheists. Unless you think we are being insincere, that is not hypocrisy.
It all would change if one could provide evidence that babies have some conception of god, or that there is some clear advantage in making atheism the positive concept that needs justification or support. But so far that simply did not happen.