Learn Italian: Questo raggiro è più insidioso di quanto non sembri. - This deception is more insidious than it seems.

This is called the pleonastic non. In Italian, when you have a comparison of inequality (using più or meno) followed by a verb, you often insert non before that second verb. It doesn't actually make the sentence negative; it just emphasizes the contrast between the two things being compared. You drop it completely in the English translation.

Why is the verb sembri in the subjunctive mood here instead of the indicative sembra?

When a comparative clause is introduced by di quanto (or similar phrases like di quello che or di come) and is followed by a conjugated verb, Italian requires that verb to be in the subjunctive. Since we are comparing the deception to an idea or an appearance ('what it seems') rather than a solid fact, the subjunctive sembri is required.

Could I use di quello che instead of di quanto here?

Yes! You can absolutely say ...di quello che non sembri. Both di quanto and di quello che are very common ways to say 'than what' before a verb in comparisons, and they are completely interchangeable in this sentence.

What exactly is a raggiro, and how does it differ from other words for a lie?