This is indeed a very interesting perspective, and going back to read it from this angle is well worth your time. Though from that initial quote on the last page, I thought that McReady was just musing on the idea of there being more creatures that came from other planets, just as any one common person thinks about such things. But as you have pointed out, it is very well possible that the creature have been in McReady from near the beginning. The argument for this possibility stands strongest on page 24 when he says “It must be a peaceable thing, in its own – inimitable -way. It never
had to, because it always gained its end – otherwise.” Though its true the confidence exuded from McReady is evident throughout the story, this comment is nonetheless one that caused me to look back over a second time. Why would he suggest such a thought, that the monster may have already gained its end. Van Wall’s response confirms this: “You’re suggesting then, that perhaps it already has the greater numbers, but is just waiting – waiting, all of them – all of you, for all I know – waiting till I, the last human, drop my wariness in sleep.” Teasing out this possibility that McReady is indeed a monster, this comment fits right it. He, as a monster, using this ability to shape-shift, is just waiting for the opportune moment, and is instead actually inside of most of the humans anyway. While this thought in the story is quickly moved from, it is an interesting thought to pull out nonetheless.