But what happened to his inner Slytherin, and the Draco inside his head? They, ironically, are the ones whose warnings (about suspicious plotting) should have the best chance of getting through to Harry.

That's an interesting point. In context of that, consider the following -- Harry is now [end of chapter 54] without protection from the Dementors, thus gone entirely to the 'dark side,' which in Harry as in most is rather Slytherin. That means that Harry is now in the perfect position to see how he's been manipulated, and act against on it: specifically, betraying Quirell and going with his first story "He made me do it." He can even attribute his attacking an Auror who thought about Moody* to the Dementors and potentially get away with the whole thing.

Just something to consider.

* I misread, but the point remains.