"Ignorance is no excuse" because if it were, any halfway-competent criminal could cultivate scrupulous unawareness and be untouchable

I don't think that follows, because they're still exhibiting meta-awareness; they're aware that there's something to be aware of which would cause them to be culpable.

It's like receiving an assignment from a professor or boss, where you believe that he has expectations that you don't want to bother to meet, so you don't ask in order to maintain plausible deniability. You might be able to claim you didn't know, but you still behaved irresponsibly with respect to his expectations.