Yes, Durkon certainly appeared to be a more powerful cleric, as a pure cleric, than Malack when he was alive, and is now a strictly more powerful character.

Yes, that could very well be read to mean that Vampire Durkon is more powerful than Tarquin.

No, we do not know any established-as-still-around character in the OotS universe more powerful than Vampire Durkon, save Xykon.