The atheist makes no claim that they can be agnostic about
First a note: the label "New Atheist" serves no purpose here, because it does not refine or clarify anyone's arguments or stance.
The vocabulary for the context for this post:
Atheism means rejection of faith-based doctrine, arguments and claims
Faith means belief without supporting evidence
Hence there is no such thing as New or Old or Somewhat Out-Of-Fashion Atheist. There is only one manner in which you can reject religiously inspired claims and/or arguments, and that is state "No, I reject that claim". There is no "New" or "Old" way of doing it, except possibly to bring new arguments into the discussion.
Second note, the Flying Spaghetti Monster is not an attempt at ridicule. It points out that whoever wants to remain an agnostic — in the complete absence of evidence — is entertaining the notion of a deity by Special Pleading. Special Pleading is not a logical fallacy per se, but it does point to an argumentative inconsistency (colloquially called "hypocrisy" or "double standard").
The main point... you want to make the argument that:
NA seems to forget that P never ascribed to agnosticism, but merely pointed out that it was NA who originally used an agnostic-like argument in (1), but then refuses to consistently follow their own argument.
You are trying to make a Tu Quoque towards the atheist. But it fails. Let me clean the fluff out of your hypothetical dialogue.
Theist: I claim to know the divine will.
Atheist: I reject that claim.
T: Why do you reject my claim?
A: Because you have not presented convincing arguments for it.
T: You cannot prove that the claim is not true. It could be true.
A: It could indeed be true. But we — you included — have a habit of not taking unsubstantiated claims for fact, even when they could possibly be true. This goes especially for claims about the supernatural. Instead we relegate such claims to the category of "fantasy".
In brief: the theist is making a claim; the atheist rejects it. The atheist is not making an assertive claim, so there is no such claim for them to be agnostic about.
Hence your argument — that the atheist is being agnostic about their own claims in a hypocritical manner — falls.
Alternatively...
If your conversation had played out a bit differently, your argument would have held.
Theist: I claim to know the divine will
Atheist: I reject that claim
T: Why do you reject my claim?
A: Because there are no gods at all, therefore there is no divine will to be known, and therefore you cannot know of any such will.
T: You cannot prove there are no gods.
A: It could be true there are no gods.
...then your argument would have held true. If the atheist brings forth such an assertion — "there are no gods" — then that is a claim that they can be agnostic about and then it would be hypocritical to strike down on agnostic arguments from the theist.
However...
...atheists very rarely make that claim. Not even Dawkins — that you referred to — goes to a seven-point-oh on his seven-graded scale, and stops at "six point nine".
Christopher Hitchens — that you also referred to — made this distinction very clear, and argued that atheism is no belief in the existence of gods, as separate from belief in the non-existence of gods, in a debate with his brother Peter Hitchens (32 minutes, 10 seconds into the video).
[The] atheist proposition is the following — most of the time: It may not be said that there is no God; it may be said that there is no reason to think that there is one.
And no atheist needs to go as far as to claim the non-existence of gods in order to achieve the rejection of theistic claims. Admittedly, it is true that when the claim that there are no gods is accepted into the discourse, then all theistic claims fall consequently, and it would therefore be a convenient shortcut for the atheist to reject all theistic claims by using the claim of non-existence. But — barring that — the atheist can simply go the slightly longer route and show that the theistic claims are unproven.
The only difference this makes is in regards to agnosticism. The claim that there are no gods kills agnosticism outright, while the position that claims about gods are unproven allows for it.
That is where the counter-argument you listed above shows up: sure... it could, possibly, be true that there are gods. But why would we — collectively — bother about that possibility, when we never bother about any other supernatural claims other than to use them as fanciful flights of fantasy that tickle our imagination?
