Sod. I hadn't actually considered that. I set them as opposites to benefit from the opposite_opinion and to ensure that you can't load up on genetic traits in the Ruler Designer to reduce the massive age modifiers the immortal traits give you. Maybe we should have an event that maims a god to test if it removes their divinity. 
One other thing - the potential section of the Walkure decision should also include has_global_flag = gods_exist, so that way you can't form Walkure without being Divine and you can't call the Valkyries without having formed Walkure.
One other thing - the potential section of the Walkure decision should also include has_global_flag = gods_exist, so that way you can't form Walkure without being Divine and you can't call the Valkyries without having formed Walkure.
Code:
potential = {
ai = no
trait = immortal_god
has_global_flag = gods_exist
NOT = { religion = war_pagan }
}