u/DARK_LEGEND-

Correct me if I am wrong,
Flugel is Subaru, not Subaru being Flugel.
What if there is a chance that Flugel might have the power to foresee the future, and he saw the best way to save the world and his lover, Satella, was to be reincarnated as Subaru? He sent his soul to Earth while his physical body remained in the Re: Zero world. So with his body parts, the witch factors were created, and Subaru(Flugel) started claiming his own power to become a complete human(Flugel?). As those his own power in the Re: Zero world, that's why when Petelguse used invincible providence, Subaru could see those hands.
There is another fact in the shadow garden, each time Subaru gains a witch factor, Subaru gains body parts there, and for some reason, unconscious Subaru loves Satella a lot(might be because Flugel's soul remembers her in that place).
The title Re: Zero doesn't align with Subaru cause when he started his journey, he got rbd, which doesn't count as starting from zero. But if we take this theory as true, then Flugel starts his journey in this world again, but in a clean state(no memory of this world), to save his lover and the world. This might be the reason Satella doesn't care about Subaru's rejection because she loves his soul(Flugel), not him.