The more trusting Harry may be an artifact of his being terrified. He got played on his dislike of the Dementors to get him in there. Once the shit hit the fan he was running terrified and taking whatever solution appeared to him. In this case Quirrelmort sounding even slightly reasonable (remember he's been talking to himself to keep the dementors off) would be accepted. I'll be interesting to see what happens when he gets back to civilization.