2) The Archangel Columbia is probably just a metaphor (or a lie) created by Comstock. His "Archangel" were the portals to other universes, throw them he saw multiple events of what could happen in the future. Like I said before, he knew about all the events in the game, he said it was the Archangel Columbia who told him those things, but it really just was the trans dimensional portals.
But only Elizabeth could open the tears, and only the Luteces could travel between universes without them. He trapped Elizabeth in a tower and executed the Luteces, so there was nobody left to tell him.
Plus, he does genuinely sound convinced of his Archangel's existence during the scene where he dies. Perhaps he had just deluded himself for so long that it started to seem real.