Pretty sure the soul is a different being as we saw she was given the memories of what happened in the body's first live through, but that demon, devil, dark spirit, or whatever it was had left a seal on the memories tied to anything dealing with him, but she's been breaking the seal bit by bit while he's trying to keep her out until she needs to remember. She had the pentagram's as a medium to see into a section of the sealed memory, but it doesn't tell you much besides her muttering something and the being clearly going to send her back in time with their contract still intact I assume.
It seems like another one of those plots where the "bad guy" is either really evil or just misrepresented I mean he did ask her what would she do if she could go back almost giving her a second chance so who knows. I'm more worried on if the sister is suspicious or not and what freaking secret is behind the elder brother ヽ(`Д´)ﾉ
Here's my take i think og sylvia made a deal with the devil or demon lord to be back on the past but i think mc is not isekaid instead she's still the og sylvia and the demon lord sealed her memories OR maybe she's the soul of sylvia from a different world they just merged into one that's why she have clear memories of the past sylvia and she's stronger now and able to summon the spirit king. The demon lord is gonna use sylvia as a tool on overtaking the human world.