How did Marika manage her dual identity during key events?
(This is all assuming they had always shared a body and would have to switch between who is in control, like in the final battle. If they were at one point two separate bodies, then the questions are no longer valid)
It would require her to have key people be in on it (like Godfrey) and even then it would be extremely difficult. Let's see some of the major event and time periods where it would cause issues:
Let's assume Godfrey knew his wife can turn into Radagon and during the battle with the giants she chose to fight as a regular champion in the form of Radagon and not use her god-like powers even tho it would have made the battle easier.
But then she would have to switch back to Marika when she addressed her soldiers after destroying her giants, at the church of Marika in the mountaintops. And if Messmer was still around back then, it becomes even stranger - who is this only other red-haired 8 foot tall guy as strong as a whole army? Is he related to the other red-haired guy?
The champion Radagon would have to hide somewhere, turn back into Marika, change clothes and show up and meet the army.During his marriage to Rennala, she would have to be in on it as well. Radagon had people wear masks near his quarters with Rennala in Caria, so possibly she knew what was up and was ok with Radagon being with her half the time. Does that mean he snuck out or teleported away regularly, to act as Marika in Leyndell so that people don't ask where she went? Or if he stayed as Radagon the whole time in Caria, then Godfrey would have to remain in Leyndell and tell all of the nobles that Marika is off busy and can't show up
People seem to know Radahn, Ranni and Rykard are demigods, yet they also know they are born of Rennala and Radagon - who to the larger populace is presented as a mere champion and later elden lord. How come no one asks the obvious question - how are they demigods if the one true god is Marika, and once again what was the deal with this Messmer guy in the war with the giants that had red hair and was 8 foot tall?
Melina seems to know about things Marika has said, even in private when talking to Radagon (basically talking to herself in the mirror?), does that mean she was also in on it and present at these moments?
Her children, at least some of them, must have known. It has been mentioned that Radagon spent time with Miquella and taught him golden order incantations. Miquella is extremely intelligent, and since he eventually used the Divine Gate, he must have some good insight into what a god is, and into the nature of Marika/Radagon. It's safe to assume one more of her children were also in on the secret.
I am just trying to imagine how she switches back and forth between two forms when parenting them, it's very strange.Morgott and Mohg might have not spent any time with her, but if they were unshackled temporarily to be used in the war against the giants, again it would have been strange that right next to them is fighting not only Godfrey, but also his wife in the form of Radagon, currently married to Rennala, the same wife who is their mother and banished them to the sewers.
It seems like a huge amount of effort to keep this all up, when in many cases she could have showed up as Marika and annihilated her enemies with her god-like power or at least subjugated them like she cursed the fire giant.
The two explanations I can think of, and are not mutually exclusive:
Radagon has a desire to exist and live a life of his own, so Marika is forced to share her time with him. He however only has his own power and can not wield the same god-like power as her, so he could not beat Rennala's armies
Marika, for reasons unknown, can not fight or is just not a powerful fighter. This would be strange as I would expect her to at least have destructive holy incantations like Miquella, but on an even grander scale. Some have theorized that all her children are just aspects she has divested of herself, so maybe after creating them all, she was left with very little combat abilities of her own?