You know, just a thought, a theory:
We now know that soul can be stored as an equipment, that Uncle's inventory and its contents got carried with him back to Earth, and that he still has the sword.
Now, if I recall correctly, what this another transferee got as his "bonus" was his sword, and I think it's correct to infer that his ability can be used with that sword as the medium, similar to Mabel's clan being able to use the sword that their ancestor got as his bonus.
So what if Sui-- or maybe even Uncle himself, planned to use that to transfer Sui's soul to Earth?
Sui could use the sword, transfer her soul into Uncle's body, then Uncle could use the Gap Space Magic to store her soul, as long as it's done within the 99 seconds limit.
The rest would be regarding whether Uncle could also store Sui's soulless body... this might be perhaps the part in which the tragedy occurred? After all, I don't think Uncle is warped enough as a person, to find and get just random someone to be possessed by Sui's. The sword is still in his possession. Uncle even has one of her dresses (or was it given by her? it's been so long). Why doesn't Uncle remember about this? Did he wipe out his memories regarding this because he had failed the attempt, or that he realized afterwards that the plan was flawed in some kind of way? Maybe Uncle doesn't know how to use the sword to transfer someone's soul, as opposed to transferring the user's own soul? Who knows.
All in all, despite of all the tragedy theories, I'm still hoping for them to get together in the end. Though honestly... what Uncle said before this is still bothering me, regarding how he went mostly solo after all of this. Maybe Sui simply thought she troubled him with this incident, and so she wanted to focus on her original mission of gathering ancient tools, which could boost her strength. Or maybe there would be a trouble with the other elves in her clan somehow...