It resolves the atheist of all need to defend their stance or support their arguments, or indeed, even have a stance or make an argument in the first place! They simply 'lack a belief' and if presented with an argument, can continue to lack that same belief.

It's not clear to me what you're actually trying to say here. If we just switch "lack" for "have" (or "hold" or "possess" or whatever) we find it says the following:

It resolves the atheist of all need to defend their stance or support their arguments, or indeed, even have a stance or make an argument in the first place! They simply 'hold a belief' and if presented with an argument, can continue to hold that same belief.

What exactly is different about the two? In either case, someone holding to such a pattern is merely stubborn or close-minded. But it doesn't actually have anything to do with whether they "lack" or "possess" belief; either way they're ignoring an argument. This seems to detract from your main point since you are trying to attach extra baggage to "lacking" belief which is equally present for "possessing belief."

This tactic is disparagingly called 'shoe atheism' sometimes, because carried to its logical conclusion, it suggests that inanimate objects (which likewise can be described as 'lacking a belief') are atheists. And yes, I've actually seen atheists defend this definition, arguing that shoes and stones and other inanimate objects are atheists.

Language is a tricky thing. There are reasonable interpretations under which you could claim inanimate objects are atheists, depending on how you define your terms. If you mean, "doesn't believe in a God," then, vacuously, anything incapable of believing (ie, an inanimate object) is automatically an atheist. If you instead define atheism as "believing there is no God," then inanimate things cannot be atheists.

The important thing to note is that these are different definitions. You can define labels in such a way that no one would choose to describe themselves with them; that doesn't make them accurate. Many or most atheists in this community at the least use the former definition, rather than the latter, when they refer to themselves. Consequently, I don't understand your consternation at the defense of something so trivially true.

Since you're talking about belief, knowledge, and definitions, one thing needs to be clear first. Defining atheism as something more than a mere lack of belief in gods would be circular. You need to start from this definition, and show why it entails something else.

Alternatively, you could try to generalize the definition based on trends which are present, but don't definitionally follow. As an example of what I mean, "fish" are "all gill-bearing aquatic craniate animals that lack limbs with digits" (according to Wikipedia). It's true that all fish have fins (with a few exceptions) even though it doesn't follow from the definition itself. However, keep in mind that we're not redefining fish, we're just observing them and drawing a conclusion about the group.

Now, I can see three options for you: you can believe me when I say that the Gostak is blue; you can say that regardless of what I say about Gostaks, you have no evidence that they are or are not blue, or you can say you do not believe they are blue. No matter what you believe at this point, however, it is more than "lacking a belief".

I don't follow this at all. There's a great many more options. For instance, it's possible I convinced myself all Gostaks are blue prior to you saying anything about it (that's about as reasonable as taking your word for it, so I don't see why it's not an option).

The simple way of understanding it is, if I were to ignore your claim (perhaps I went momentarily deaf) then what questions could you ask me to test my belief? And how would they be different than if you had said nothing? (Hint: they'd be the same).

For example, if you asked me: "Do you think Gostaks are blue?" (without prompt earlier) I would say, "I don't know, I don't have any reason to think so." And I'd say the same thing if you asked about red, green, yellow, or any other color.

If you asked me, "Do you think Gostaks are not blue?" (again, without prompt) then I would still say, "I don't know, I don't have any reason to think so."

Suppose you then made your claim: "Gostaks are blue." If asked again, how would my answers change? I expect they'd go something like this:

"I don't know if Gostaks are blue, but he says they are."

"I don't know if Gostaks aren't blue, but he says that isn't true."

So, still just a lack of belief. I now have belief about your belief ("I believe that you believe that Gostaks are blue") but unless I have some reason to trust your judgement (and in this case I don't) then that doesn't affect my belief at all.

You can, for example, believe that I have not provided enough evidence to overcome your skepticism when it comes to Gostak coloration.

This is a positive belief about your belief, not a positive belief about Gostaks.