Learn Russian: Вы всё-таки не замечаете обмана?

Вовсе не (not at all) and далеко не (far from) are used to aggressively reject a false assumption (e.g., "I am not at all tricked"). Here, the speaker isn't refuting an assumption about themselves; they are expressing disbelief about someone else's ongoing failure to see the truth. Всё-таки means "still," "nevertheless," or "after all." It intensifies the negative question by highlighting a broken expectation: "Despite all the obvious clues, do you still not notice?"

Could I just use всё ещё (still) instead of всё-таки here?

You could say Вы всё ещё не замечаете обмана?, but the emotional tone changes. Всё ещё is emotionally neutral and simply states a fact about time (the action continues into the present). Всё-таки adds a strong layer of contrast, contradiction, or frustration, which perfectly fits a question where the speaker is astounded that the listener hasn't figured it out yet.

Why is the word for deception in the genitive case (обмана) instead of the accusative (обман)?

This is the "genitive of negation." When a verb is negated in Russian, its direct object often takes the genitive case instead of the accusative. At the B2 level, the rule of thumb is that the genitive is strongly preferred if the noun is abstract (like обман - deception/fraud) and if the verb is one of perception or thought, like замечать (to notice), видеть (to see), or понимать (to understand).

Why use the imperfective verb rather than the perfective ?