Here's where I disagree with you: I don't think Harry "saw through" Draco at all (his judgement on Draco throughout his entire Hogwarts career seemed to be VERY surface level - based more on the way Draco carried himself than on anything deeper)! As Lucius would be able to present a better veneer (and it was the veneer, not the conflicted core, of Draco that Harry noticed) I think that Harry would trust Lucius more than Dumbledore...
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William//Slytherin