Is it possibly to be an Atheist and at the same time fit under the Abrahamic label? In the same way there are atheist Pagans and atheist Dharmics and atheist LHPs and etc.
And is it possible for an Abrahamic religion to be polytheistic? Or is monotheism required?
As far as I know, monotheism is a requirement, at least for the "Big Three," in the sense that polytheism is completely incompatible with their theological teachings.
Atheism? I don't know. First of all, I am actually skeptical about true atheism. Most of the atheists I've known who are still at all interested in associating with Judaism are not really atheists, they're just strongly agnostic-- in other words, they aren't 100% convinced there's no God, they just tend to think He doesn't exist as such-- or they're not atheistic, they just don't believe in a personal God.
But even for fully avowed atheists, there do seem to be a number who claim Judaism as a mantle. They can get away with this at all because Judaism is more than a religion, it's a socioreligious ethnicity (combining religion, nationality, and cultural ethnicity in one, inextricably), so even though they are trying to ignore the religious aspects of it, there are still cultural aspects that they can grab on to.
And yet I remain skeptical. The cultural and religious aspects of Judaism are ultimately not severable, and I have never yet encountered a Jewish atheist who participated fully in Jewish society and Jewish practice. When you take Jewish practice, ritual observance, and study out of the picture, you aren't left with a whole lot-- what is left is less of a framework for daily life and living, and more of a hobby, I find.
Atheist Jews remain Jews because under Jewish Law, if one is born to a Jewish mother or converted to Judaism properly according to Jewish Law, one remains Jewish for life, regardless of one's actions. But to what extent they can be said to actually be living a Jewish life? That I don't know.
And I definitely don't know about atheists in Christianity and Islam, which have fewer cultural aspects and no national aspects, and less theological room for flexibility. It seems like "Christian atheist" or "Muslim atheist" would be difficult labels to make mean anything.
