This doesn't seem all that conclusive to me.

In fact, I'm pretty sure that events did not happen as Harry deduced them. In canon, Snape overheard the prophecy being given. This seems to match up with what Snape says in the chapter formerly known as 77:

I thought I had merely happened to overhear it, when in truth it was I who was overheard.

Therefore I suggest that Dumbledore did not tell Snape about the prophecy. I'm not sure why or how McGonagall knows: possibly, in a departure from canon, she was conducting Trelawney's interview instead of Dumbledore himself.

The beginning of Harry's line of reasoning is that Voldemort was not terribly eager to kill Lily Potter. This seems likely to be true, but is also not indicative of anything. It's a mildly strange thing to imagine, but we imagine strange things all the time.

Quirrell suggests that Harry can see Thestrals due to his memory of that night, which would suggest that it's a true memory. But Harry seems to think that he saw the Thestrals because he has realized that Dementors are death, so that he has "seen death and comprehended it" in a more literal sense.