If Fell Zelkov was somehow rescued and lived on the Somniel with everyone else, do you think he would eventually have a moment where he lets his guard down and breaks down crying since Ivy is actually treating him as a human being rather than as an object? And how confused would Ivy be at his behavior and the way he tries to flatter her?
Let me try to summarize my thoughts on Fell Zelkov...
As with the other Fell characters, I think of him as being some of Zelkov's worst tendencies pushed to the extreme.
I get into it some in Broken Mirrors that Fell Zelkov is empty and numb inside. He takes depression naps. He has no hobbies. Fell Ivy is all he has, and his world revolves around her. He loves her and hates her. In my (justifiably unpopular) fic, The Rabbit and the Snake he refers to her as his anchor. This was a very deliberate dual meaning. She weighs him down, but also keeps him from losing himself completely.
Zelkov (both of them) needs to be needed. He has to prove his worth to justify his existence. In their bond conversation, Leif says, "Perhaps you could teach me how to make medicine. It would be useful for me to know." Zelkov replies, "Absolutely not. Making medicine is my role. What would I do if you replaced me?"
Fell Ivy needs Fell Zelkov, and she always will. She needs someone who will be entirely devoted to her and tell her she's beautiful and be an enabler and put up with all her shit (and also someone she is dtf with). But I think most of all, she knows that he needs her and will never leave her no matter what — except in death. And my understanding of it is that Zelkov's (and Kagetsu's) death is what pushes her to want to raise the Fell Dragon.
In The Rabbit and the Snake, when he meets regular Ivy, Fell Zelkov immediately falls for her, but actually what happens is that he's deeply envious of the relationship between Ivy and Zelkov where they respect each other and truly care for one another. He wants that for himself, so he kidnaps her and tries to replace regular Zelkov. Eventually he realizes his plan was never going to work, and Fell Ivy is gone because of his actions. The one who needed him is no longer there. He gives in to despair and kills himself (although he was actually already dead and Corrupted).
So I don't think Fell Zelkov is any kind of saint. If he was rescued somehow and ended up on the Somniel, I think it would cause a LOT of drama because he wouldn't know how to act. He would latch onto Ivy, once again trying to find that need and sense of purpose, but she wouldn't need him in the same way that Fell Ivy did. She would be put off by his flattery. There's a reason he tries to trick her into thinking he's her Zelkov, rather than trying to create his own relationship with her (in the fic). Also, Regular Zelkov has enough trouble accepting others' kindness. As a Zelkov with somehow even worse mental health, I don't think Fell Zelkov could ever accept that someone could truly care for him in an un-self-interested way.