Re: One Sun, Many Sungods?
I see three ways to go about this.
One is to have one overall pantheon that is basically static but whose members are referred to by different names depending on the religion. So the all-father god would be variously known as Odin, Zeus, or Jupiter, depending on where you were, while the sea god would be called Aegir, Poseidon, or Neptune. You could change this up a bit and have Poseidon/Aegir be more powerful among practitioners of the island-peoples' religion than in the religion of the mountain-people. But overall you would have one pantheonic structure.
The second, weirder, way would be for religion to be mortals' uncomprehending perceptions of incomprehensible divine forces. Because mortals can't possibly understand the true nature of these forces, their will perceives them to take the form of gods and goddesses, and the perceptions, if not the forces themselves, are shaped by religion. As a religion gets stronger in followers and faith, the divine forces it relates to take on a more and more definite form and increase in power. So while two mortals perceive two different sun gods, those gods are nothing more than different perceptions of the same force or even different forces. In game terms this would be practically meaningless, it just means that mana is mana no matter which cleric you're playing.
The third way, which I think of as "JLA theology", would be for each pantheon to have its own structure and members who may or may not have analogues in other pantheons. They would play a lot like high-powered superhero/villain groups, usually teamed up with one another but on occasion you might see the Divine League of Ambria join forces with the Divine Society of Ambria to defeat the Legion of the Divine. There might even be reason for the sea gods of all the pantheons to join up to fight all the sun gods, though ordinarily there would be something that discourages that and causes gods with similar powers to join separate groups.