This ability ONLY triggers when Leveticus is killed or is sacrificed. Same wording as Assassinate and Finish the Job. He does die. He just doesn't stay dead.
So, timing matters. I would say that, under the Rules As Written, you would get the points for Assassinating Leve in Scenario 2 and 3 above.
I would also say, with Rules As Written, that you would get to drop a Scheme marker.
While Leve does not end up dying, he must be dead at some point, or Pariah would never trigger.
If the wording was
"When this model would be killed or sacrificed, it is not...."
I would agree that you never get 4 points for Assassinate and you cannot use Finish the Job. As they are currently written, and how the rules for simultaneous actions work, that the player in control of Leve can be dead when he wants to be, and then cancel that death afterwards.
However, I am perfectly willing for that interpretation to be wrong or unintended. It probably shouldn't work that way. But, right now, at a tournament, that is how I believe the rules would work.