I don't think it's confirmed yet that Pastor A's power comes from her Christian belief. It might, though perhaps people just have natural mage inclinations regardless of their religion.
At the least, however, her being Christian rather than having converted to any of the other possibilities hasn't prevented her from having that power.
As for the "benefit of switching beliefs", with the risk of delving into a too serious topic, I don't think choosing a religion is such a logical affair for most people. You either believe or you don't.
That's true.
It just seems to me odd to have Christianity have disappeared so thoroughly in only 90 years, when many people have hung on to their belief through many other disasters. If the other gods were actually manifesting and actively helping people while there was no sign of the Christian god(s) doing that, that might provide some sort of explanation, even though belief's not generally based on logical thinking. But if that's not what happened --
Maybe it was political? The conversion to Christianity in that part of the world was in large part political in origin, after all; and the belief may have followed practice, as it may have done in the earlier switch.
Also, personally, I wouldn't call having one's conscience trapped in a troll body for 90 years a "benefit", but maybe that's just me
She seems to be protected from the worst impacts of that. She's not begging for help, like the other trolls; and, if we go by her conversation with Reynir and Onni in the dream world, she didn't seem to be in pain or in despair, or even to realize that she is in a troll body -- just to be unable to clearly remember what happened, and to have mostly lost her sense of time, so that she doesn't realize it's been anywhere near as long as 90 years.