Luffy's red hawk being his body hardening and then causing friction has always been headcanon, Bonney's abilities are the definition of stretch and Law turning the sound off has nothing to do with surgery
I don't know about that headcanon about Luffy.
Red Hawk made zero sense at the time, just like Gear 4 made zero sense at the time.
Now we know it was some sort of mid-awakening of sorts. Both Doflamingo and, specially, Kaido, noticed G4 Luffy could do things that shouldn't be possible just by having a rubber body like changing the trajectory of his fists in an angle mid-punch. I have been commenting on how that was bullshit since Dressrosa.
At Fishman Island, Luffy had already achieved G4. It's not a coincidence that he started showing some occasional fire moves, or "running so fast he created a trail of burnt marks on the floor" on Punk Hazard, similar to the fire trail he created on G5.
--
Bonney's abilities are the definition of stretch, indeed, but that's her fruit.
It's limited by imagination and it could actually become weaker as she ages, or stronger as she gets a clearer picture of the strength of a real top tier to imitate.
--
It can just be anything like "a sound proof room" or any stretch Oda wants to come up with.
Edit: It's also why G4 has that smoke thing on his shoulders.