Based on some details here and there, and a suspicious sudden awakening, scenarios like SD possession aren't completely unreasonable, but it doesn't mean they are proven atm.
When Mael's form changed to he faceless six winged angel with a long gown, I wrote a post about the possibility of SD possession. That she might be the only one to withstand the presence of 4 decrees. At that time, Mael was among the few angels with a real body, and everything seemed to work fine. But it was completely different from what Nakaba had in mind.
In early 4KotA, it was like Nakaba intentionally doesn't wanna show Ellie's face for some reason. I was daydreaming about a big change in her looks, specially the eyes. As if Nakaba has planned something big. All hyped up for nothing.
Any form of SD's presence is like a dream come true for me. It's the perfect opportunity for Nakaba to compensate for all the mistreatments. But it's too soon for thinking of it as granted.
The same goes for those who criticize Nakaba so early. If that's the scenario, we can't judge its pitfalls unless we are at least shown some basic concepts. Like SD willingly destroying her daughters human body by using it as a vessel. Knowing SD should be either dead or imprisoned within Aldan, she can't even be the one to decide her own fate. And Merlin always comes up with plans to push her plans forward, and she deeply cares about the life of her comrades even though she deliberately puts them on harms way! A paradoxical behavior just like Merlin! Thinking of any SD resurrection without Merlin's involvement just doesn't make sense after Cursed by Light.
So both sides, remain calm and wait for more data before final judgement. The story is far from ending, and there are still lots of opportunities and potentials for Nakaba ... ... ... ... TO WASTE!