The velocity of 5.56 makes many of its loads less likely to fly through a bunch of walls compared to 9mm. The 5.56(bullet dependent of course), is more likely to break apart. But any cartridge capable of killing someone is also capable of flying through a few dry wall sheets.
Also, you mentioned this:
That is very wrong. The permanent cavitation caused by rifle rounds at close self defense distances is vastly more likely to kill someone quicker than 9mm. The rifle cartridge is more likely to cause sudden hypovolemic shock. And it’s also less location dependent. For 9mm, you have very small targets for instantly putting the bad guy down. With a rifle, there’s more wiggle room. In fact, just having a rifle bullet pass in close proximity to the spine can cause temporary incapacitation. The pressure through the tissue/fluids during the temporary cavitation can travel through the subarachnoid space around the spinal cord and cause the spinal cord to smack against the vertabrae. Causing the bad guy to drop, even if not permanently. That can, of course happen with the temporary cavitation of a pistol cartridge too, but their temporary cavitation is much smaller.
Rifles are way easier to cause immediate incapacitation than pistol calibers at self defense distances.
That said, I do think PCCs have their merits in home defense. They are much quieter, even as you go down to the shorty short barrel lengths that are desired in some smaller homes (like those common in New England). People rarely balk at having a pistol in a nightstand for home defense. Yet you mention a PCC or braced pistol, and woah, all of a sudden the cartridge won’t stop someone?