(...) Agnosticism is not valid practically because it assumes "God exists" is a magically different kind of question from everything else and therefore "Magically unknowable" has a kind of validity that it doesn't have in other cases. That is not accurate and is special pleading.
I think it was a great idea of the OP’s, starting these two threads about the actual meanings of the terms ‘agnosticism’ and ‘atheism’. One often comes across these disagreements about which is a more valid position to hold, atheism or agnosticism, and which kind of atheism is best, soft or hard-boiled or poached. I believe it simply boils down to semantics, these disagreements, in nine cases out of ten ; and clearly sorting out this semantic issue will probably (or so I think) make most of these disagreements disappear.
Here is how I define the term ‘agnosticism’. The agnostic admits that one does not have sufficient information, sufficient evidence, to form a conclusion about the God question ; and further, lacking this information and evidence, one takes the default position that there is no God. That, far as I can see, was Huxley’s intent in coining that word. (Etymologically speaking, the word ‘agnostic’ refers only to what one knows, and therefore the position I described would accurately be described as agnostic-atheist. However, meanings of words don’t necessarily perfectly reflect their etymological origins. I choose to follow the nuance that Huxley, I believe, intended for the word.) Of course, usage of words often evolves away from their original meaning : but in this case, in the case of the word Agnosticism, I think what we have is not so much an evolution of the usage, as simple confusion. If one were to go by Huxley’s intent, then agnosticism is no different from ‘soft atheism’. A position that appears perfectly reasonable to me.
The Agnostic -- as I’ve defined the term just now -- treats God no differently than the invisible faeries that frolic around in my garden, undetected and undetectable ; and no differently than Russel’s teapot (or Dawkins’s dragon). The Soft Atheist -- I know, ridiculous term that, “soft atheist”, it is as if the term simply begs to be ridiculed, all the more reason to use the term Agnostic instead -- I was saying, the Soft Atheist or the Agnostic sees no evidence for Santa, or for God, and therefore, pending evidence, believes in neither.
If we go with this particular definition of the word Agnosticism, then no, agnosticism does not assume that “'God exists' is a magically different kind of question from everything else". It does not hold that “'Magically unknowable' has a kind of validity that it doesn’t have in other cases". No, it isn’t special pleading. And yes, it is perfectly valid.
True, there are dictionaries that define these terms differently. True, there are people who use these terms in different senses. People disagree about what these words mean, that is fact. That cannot be wished away. So what is one to do? The common-sense answer is this : One recognizes that this is merely a semantic issue, no more and no less. Therefore, instead of saying things like “Agnosticism is valid” or “Agnosticism is not valid”, it makes sense to simply define these terms first to everyone’s satisfaction. That first and foremost. Once one has done that, I wager we’ll find that the underlying disagreement itself has disappeared, in nine cases out of ten.
For instance, in this post I clearly formulated my own position re. God-belief. You may agree with my interpretation of Huxley’s intent, and agree to join me in calling this position Agnosticism, as I do. Or else we may both agree to move to an etymologically-derived interpretation instead, and refer to my position as agnostic-atheist. Or else we may, provided we can stop ourselves laughing at the absurd picture that those words convey, call those who hold this position Soft Atheists. If we don’t agree to any of these terms, we can then clearly spell out the position that we’re talking about to each other, make sure we both understand what we’re saying, and call it simply “my position” or whatever else we want, for the space of our discussion. Once we’ve done that, we will (very probably) find that we don’t really need to discuss anything at all, that we’d been in perfect agreement all along.
Again, please read my 
But when applied to the god-question, that word takes on an added nuance that it doesn't have when applied to one's spouse's fidelity. Because Huxley.