Oho! That could be useful. Law is definitely a planner, not a "run straight into the pack of enemies and start fighting" guy like Luffy, so he'd be all down with skillfully using subterfuge if it helps in the long run.

I suppose it depends on whether the Null would be fooled by anything he can do - taking a heart means there is literally not a heart in the body, even if they're still alive, so measuring a pulse would make a person appear dead. Or, he can easily lop off a head which would easily make someone appear dead even though they're still alive, just in pieces. He can even do it to himself but fixing it requires getting all the pieces back together again.