Here are two different ways that I have looked at the gods in a couple of
the campaigns that I have run (though not in Birthright, yet).

1. From Piers Anthony's "Incarnations of Immortality" - The Office holds
the Power not the Office Holder. The Sphere of Influence that the god has
is more important than the individual who controls that Sphere of
Influence. Andurias died and was replaced with Haelyn. The clerics
worship the Law Maker and the God of Noble War, it does not matter what
his name is. If some clerics want to call Haelyn, Andurias then so be it
he is already known by different names in different areas.

2. From Michael Scott's "Tales of the Bard" - Faith Lends Substance. A god
does not fully exist without the worship of his followers. At Deismar the
worshippers of Andurias saw their god apparently destroyed and switched
their worship to his champion this caused Andurias to fade and Haelyn to
emerge. By this law anyone can become a god if he/she has enough worshi
ppers. If the idea of Andurias were to re surface then the god would also
(if enough worshippers existed.) I usually have a place IMCs where the
essence of "dead" gods reside (I call it the Void or sometimes Sheol).
This is also the place where Demons and such that are killed on their home
planes go once their the essence is in a sort of non-aware state, sort of
like suspended animation.

Here is my favorite passage from Demon's Law it has direct mention of the
Faith Lends Substance Law,

The war with the Culai was long and bloody, with the technology of the
First Race ranged against the numbers and determination of the human folk,
but that is another tale in itself. It was principally fought in and
around the streets of the City and many of the battles were planned and
commanded by Buiva himself. His name became a war-cry of the humans, and
soon the Culai grew to loathe the buzzing of 'BUIVA-BUIVA-BUIVA' as it came
up from the City to their stark, white-walled houses.

The tide of war had swung in favor of the humankind when their commander
died, not by treachery and not by violence. He died simply and peacefully
in his sleep on the eve of what everyone, both human and Culai, knew would
be a decisive battle. News of his death rippled through the human army
like wind across a field of wheat, but paradoxically, instead of
disheartening them, it only served to urge the people on to greater
efforts, and before the battle all of those present - and they numbered
many, many thousands - bowed their heads and prayed for Buiva to aid and
guide them.

And Faith lends substance.

And towards noon, when the battle seemed to be going against the human
folk, a single warrior dropped on the battlefield, a thin sliver of metal
buried in his chest, and called aloud for his old commander...

And Buiva returned.

In the midst of battle, a creature of fire and bronze, of death and
destruction - of WAR - stalked, rallying the human kind, striking terror
into the Culai. A new god had been born...

Both of these ideas could be applied to Birthright. I wouldn't mind
knowing how other DMs view the gods.

Chance is the Demon's Law; ... , against it the Gods themselves are
powerless ...
Demon's Law: Tales of the Bard (Michael Scott).