I think you're underestimating how guessable things can be if one pays attention to clues. I can't remember for sure, but I think the possibility was in my mind since I first read the note about how Harry should have noticed his confusion about the story.

The list of evidence I gave in favour of that position in a Hatrack forum discussion back in November 2010 was simply that
a) We're told that Harry should have noticed something off about the story, but he didn't (simplest thing he should have noticed is: how the hell do people know what Voldemort tried to do, if there were no living witnesses)
b) Avada Kedavra normally leaves no mark on the victim. Whatever was done on Harry left a lightning-shaped scar.
c) Avada Kedavra kills anything with a brain and never rebounds. This time it's supposed to have failed to kill Harry and to have rebounded onto Voldemort.

All of these were well known before chapter 43.

Since then we can add the evidence of:
d) Bellatrix indicated she was given Voldemort's wand before Voldemort went to the Potters' home -- simplest explanation was that Voldemort didn't want his wand taken away by Aurors when his physical form was destroyed, and so he knew his physical form would be destroyed.
e) Lucius Malfoy seemed to believe Voldemort resides inside Harry's body, which is a rather odd conclusion to arrive at, unless he already has reason to suspect Voldemort wasn't attempting to kill Harry.

These pieces of evidence are not in chapter 43 either.

What evidence are you talking about that's in chapter 43?