A hint for those who also got this bug as I did and being afraid what kind of impact it has on the final fight. Without Tarquin returning to the Lady Vengeance I nevertheless went on playing and just was able to finish the game.
Just before confronted with the whole gang of Lucian, Dallis etc. Tarquin appeared again and performed the praying with Maladie, Gareth and Almira. So - at least you don't lose the advantage of using source skills for free. (No consumption of source points).
