- Location
- Greece
You can be one of the Lady's Champions (Grail Knights) while also worshipping other gods and so be eligible for the throne. Indeed, it would be perfectly possible to be blessed by another god. I think there are canon Grail Knights who have Mannan's blessings as well as the Lady's.
Humans in the Empire don't have to be faithful to a single god, indeed the only people that do are fanatical Sigmarites and the mainstream of the Cult burns them at the stake for it. I think almost everyone worships the entire pantheon in respect to their sphere of influence. You pray to Morr when someone you love dies or you have a portentous dream and to Shallya when they're sick, to one of the war gods when you go to war, to Verena when you're curious about something and want to learn more or fall afoul of the law, or to Sigmar when you interact with state institutions.
As I said, polytheism is accepted, the lady just has constitutional primacy in rulers, much greater than Sigmars. You can worsip someone else too, sure... as long as the Lady comes first.