He escaped from the hanging because the man doing the hanging placed a hook which somehow displaced the weight of the rope tightening on the neck and thus he didn't die from the hanging. Then in order to avoid having a pulse, he ingested a chemical which lowered his heartbeat to the point at which it was undetectable. At the cemetery, the tombstone was made of sandstone and it was pre-broken and then glued together so that when Blackwood escaped the coffin, he could easily break it and make it look like he broke stone.
