Got any evidence of that?
That... he's... um, dead?
I mean, what we have is that Price said in his forenotes that the manuscript burned and that McClellan hadn't finished recreating it, and we have a manuscript that ends at Seven Pines.
There are two possibilities.
Either:
McClellan wrote two manuscripts, one of which was much further along than the other and included the Antietam material, but the further along one has thus far disappeared after Price used it as a source. For the other one he stopped at Seven Pines for no adequately explained reason.
Or:
McClellan wrote only one manuscript, the one we have located, and Price used it as a source up until the point where it ended (with McClellan's death) after which he reconstructed the rest of the material in the book.
As it happens from a brief check
the last page of the manuscript appears to be a direct match to the first paragraph of
page 378 in the published book. This strongly implies that the manuscript Price used is the manuscript we have now, and obviously since it ends there it can't have been the source for the whole book.
In the rain? Having just gotten over Typhoid Fever?
He had typhoid in December 1861, and this was September 1862. Unless I've missed a particular bout of the fever he suffered from months later I'm not sure that qualifies as having "just" gotten over it; even if he'd suffered from it in July it would be a stretch.