Tamriel is a world that is capable of holding scientific truth, historical truth and personal truth, truth conflicting or otherwise. Cannonicity is always the wrong argument as it is possible to provide a truth that encompasses all these 'truths' and creates a world in which each can be derived.
The stories of Ar'Kay the God and Thu'whacca the Yokudan God of souls are the prime example of this flexibility. When attempting to merge these two contradicting visions of the same god it becomes clear that the Breto-Nedic version is allegorical for finding a purpose in dead.
As such it's important to realize that it's people who believe the god of righteous rule allies the prince of plots. Like the story of Ar'Kay the God it should not be taken too literally. An alliance between Stendar and Boethia can be seen as the merger of ruthlessness and righteous rule. Which frankly said isn't even the most original idea, Machiavelli hammered out the archetype for this kind of behavior about 500 years ago.
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Needless to say, this applies as much too the Deep Ones from Hack Dirt as it does to Ius, Animal God.