The fact that so much of Harry's beliefs and actions are based on the assumption that speaking in parseltongue ensures that the truth is being spoken bothers me.

Harry believes this because 1. Quirrel told him. 2. He is seemingly unable to lie to Quirrel when speaking in parseltongue.

The first point is obviously something to be extremely skeptical of and, while the second appears to be strong evidence, it very strongly reminds me of

"I want to give you a drop of Veritaserum," Draco said. "Just one drop, so you can't lie, but not enough to make you answer anything..."

Of course Harry thinks he can beat Veritaserum due to his Occlumency training but this has never been put to the test and, even if it is the case, it has been established that there are things in the magical world that can be used to unilaterally manipulate a person's ability to lie.