The inefficiency of H&C’s attack against Hermione’s mind is not evidence of a “simple mistake” on his/her part, but rather exceptional cleverness. Note that this attack has replaced something that would be detected (Legilimency) with something that cannot be (Obliviation). I myself take this as further evidence that H&C is Quirrell.
Were there other mistakes you had in mind?
Assuming Quirrell is Voldemort, he presumably had years of access to Lucius’ mind (if he regularly required Lucius to drop Occlumency barriers). At the very least, we can assume he has an excellent mental model of how Lucius behaves. The plot therefore doesn’t seem like too great of risk for Quirrell, particularly when we consider that Lucius is about to discover Harry’s progress in turning Draco. Quirrell can safely assume that Lucius will react in a way that will pull Draco and Harry apart when he discovers this, and will therefore be less inclined to trade Hermione for something of Harry’s.