When Subaru Returns by Death, apparently the people around him have flashes of the alternate universes, sometimes - feeling that something happened though it didn't (because it happened in a parallel RbD timeline).

When Subaru meets Satella, he feels that he loves her, as if they had spent a lifetime together - yet they haven't.

So, the logical consequence is that she is either using mind control on him OR that Subaru is feeling what he felt in an alternate timeline.

But Satella was only ever free to roam the world about 400 years ago, and without RbD Subaru would have died on his first day in the new world, by the hands of Elsa.

Unless...

Subaru was summoned TWICE. And he is indeed Flugel. Just, Flugel was himself FROM AN ALTERNATE TIMELINE.

I'll explain with more clarity:

We have Subaru1 and Subaru2.

Subaru1 is going off his day as usual, gets transported to the other world by a young Satella, who is literally experimenting with her powers; they end up becoming friends, and Subaru1 actually lives the stereotypical Isekai story with his main heroine being Satella herself, during which he gets pretty OP (like every isekai character ever) and gets nicknamed Flugel by her.

Shit happens, Satella goes crazy, Subaru1, the Sword Saint and the Dragon seal her, and then he finds himself ALONE in a dark fantasy world, without his love and dying a grim, lonely death, not having been able to figure out how to mitigate the Witch Genes influence.

Satella hates being sealed in a cage for eternity, and while Subaru1, literally the only being who still loved her, died unable to help her, she decides to make her love return to this world to put an end to her suffering.

Thus she summons Subaru2 from another timeline, gives him immortality and literally begs him to kill her, while still loving him like crazy (literally) and fondly remembering the times they spent together in the past, which Subaru2 can now only remember through a feeling of love so strong that binds his self through all the possible timelines.

Or Satella simply brainwashed him.

Your call, my fellow Re(:Zero)dditors!