So from my understanding the pantheon trinkets are a once a week chance on personal loot, you cannot bonus roll them. You can either get it from normal / heroic / mythic, but you only get 1 "roll". They always start at 940 regardless of the difficulty it dropped in. So killing Argus twice doesn't help you. And the Boon is on the same roll as the trinket.
You kill argus:
System check: Do you get trinket loot: yes/no
System check: Do you have trinket? yes = you get boon no = you get trinket
Any know if the above is correct? I have no blue post to back any of it up, but from what I've seen and gathered this seems to be the general consensus.
*Updated*
Legendary and Epic are separate personal push rolls. You can get both at the same time. Not confirmed if you can coin them.
Blessing of the Pantheon appears to be a 100% drop now, whether or not you have a pantheon trinket to upgrade.