Hmm... Average adult human typically has a skull 16-18 cm wide, with a depth (distance from forehead to the back of head) of 20-22 cm. (6.3''-7'' and 7.9''-8.7'' respectively.) Skin, hair, [i]et cetera[/i] would only add to the measurements... In other words, human skulls are pretty big things, and their jaws would have to be capable of opening up pretty wide to accommodate. And swallowing (I actually didn't go that far, I just pictured them taking half a head between their jaws) would be an entire different matter, especially since it has been stressed that they cannot expand their jawbones like snakes do... Neck-wise it would be okay, it's only jawbones that are questionable.
As for the yth width matter (aside of head) - I understood that the ten inches was at their widest. My statement was mostly derived from the fact they appear to be have fairly large heads, followed by three or four inches until the arms, which have been stated to be as thick and strong as average human arms (my own upper arms are maybe three and a half inches thick, so let's go with that for simplicity) begin... Well, there is not too much yth left after the arms. From the back of head to the underside of the last pair of arms would be over a foot. You'd also have to take account of the fact that the long arms make their upper bodies very massive, and accordingly, the hind body must be quite powerful to be of any use while moving or rearing up... (Edit: Okay, I tried to outline something based solely on the dimensions... There is clearly not enough yth left for a proper hind body. Edit2: Also made a basic sketch of one completely disregarding the given dimensions and simply relying on basic descriptive guidelines. Will clean it up and upload it later.)

...I have a vague recollection of wanting to say something more before I went to sleep, but it doesn't come to me at the moment.