Did they go back to the site and pick up the bodies which were still floating, i.e they found J.J.A body floating and assumed he was crushed by a smoke stack? does anyone know how that works?

By wallets or initials carved in their clothing.

Thw White Star Line commissioned 4 seperate ships to return to the site and surrounding areas to find bodies. They found 328 combined. The ones that were identifiable were logged into a system they had, and usually identified by their personal possessions on them.... the ones who were too damaged to figure out who they were were also logged into the system, but without a name. They were covered in sacks, with a weight tied to them, and thrown back into the ocean.

About JJ Astor, he was found, barely identfiable. They only knew who he was by his cufflinks, I believe it was. They had his initials on it. His face and body was completely unidentifiable. Once they realized WHO he was, they saw the need to put a reason to his appearance, and assumed a smokestack falling on him would be the only way someone could have that much damage to his body during a sinking ship.

What I'm getting at it, it was never PROVEN that's how he died. Then again, they didn't have a great forensic science back then either, but, anyhow.... it was assumed he was killed by the smokestack, not proven. :)