Further, that Quirrell is mischaracterizing Hermione's death as a failure of preparation, when in fact she could not have been prepared, because her enemy was an unknown meticulous assassin who could readily counteract every preparation she could make

She could have left the school the same way Longbottom did who got his "Outstanding".

Note first that Quirrell's perspective is informed by his conversation with Hermione on the night she returned from the Wizengamot trial. In his view, her decision to stay involved, knowing the dangers, means she assumes the burden of her failure (as represented by her death).

Rereading that conversation it feels like Quirrell's position is: "I know you don't like me. I don't want to you to be here and I give you the opportunity to leave. I need a decision tonight." It's all said with plausible deniability but Hermione was already thinking that Quirrell was evil enough to ask him: "Are you here to kill me?".

In particular:

No, Miss Granger. Your departure will take time for me to arrange, and I have less time left than you may think. This decision may be painful for you, but it should not be ambiguous; much weighs in the balance of these scales, but not evenly. I must know tonight whether you intend to go.

Killing her is also an example of "arranging her departure".