As to an audience always knowing the cards are hidden in the hand, this is just simply not true.
Unless you're working for young children or those requiring educational support, I would categorically dispute this. To be fair, some audience members may conclude that they come from the sleeve or that some gimmickry is involved, but if it is demonstrated that the cards are genuine and the sleeves are rolled up, any spectator of average world-knowledge and intelligence knows that the cards are hidden in the hand somehow.
Never forget that what we do with our hands is not magic, it is deception. Magic happens in the mind of our spectators.
In my opinion those two sentences don't quite agree with each other. What we do is not necessarily deception. Yes, magic happens in the mind of our spectators, and what we do is provide the context in which they can create this magic. However, we can do this without deception. I believe a hypnotist (displaying a genuine, non-deceptive skill), or even a juggler (again, without deceit) can create that moment of amazement, astonishment, or, to put it another way, magic.
And did someone really just compare a cut to Derren Brown mentalism? haha
Well, I said a "card flourish", not necessarily a cut. And, I wasn't comparing any card flourish with any Derren Brown effect. I specifically picked out a handshake induction, because, theatrically, both the flourish and the induction can serve the same purpose. After the narrative of the act has begun, the audience will seek justification for believing in the character that the performer purports to portray. If your persona is a master sleight-of-hand artist, then a complex flourish will definitely prove your credentials, as a handshake induction would if you present yourself as a mind manipulator.