So a post game discussion as it where.
Lets start with this, a Cleric's power is derived from devotion to their God. The amount of power is based on the level of faith and effort they have put into serving their God. Shadowheart has for years been a devote follower and even aspired to be a holy warrior for Shar.
However her realization she has been stolen causes a crisis of faith. THIS is where the game and the storyline can get a little screwy.
You see if she abandons Shar she should, by lore, instantly loose access to her spells. The power that granted those to her has been abandoned. Yet she retains them.
Worse yet, the power does not change. The "theory" is that she has become a devote of Selune and that the Lady of Light has come her benefactor and thus providing her power. Would this not mean then she should instantly be changed from a Trickery Domain to either Light or Life?
Oh wait the convolution gets worse. Recall what I said above, the level of power a Cleric has is based on the level of devotion. Well Shadowheart is, by her own dialog, in a massive crisis of faith. Further she has not in ANY WAY proven herself to Selune or the Priesthood and thus should have no or VERY limited power. She should essentially right now be back to level 1 as far as spell access.
From a pure lore/plot point of view there is a work around. Selune wants the Absolute stopped at all costs. To this end she see's fit to make sure Shadowheart's powers stay in tact to help defeat the Absolute. She there by works behind the scenes, working for her own goals, not based on the devotion of the "cleric". This will result however, once the Absolute is defeated, in Shadowheart losing her "power" and needing to make a live choice. Will she become a Cleric of Selune, thus starting over, or will she become something else or just retire?
I though, as a LONG time GM, that this was an interested quandary.