Well...that's the first time I've seen the story start out from the perspective of "those left behind" after an isekai reincarnation. I hope Charolotte somehow makes it back to earth to see her old kouhai eventually...why else start out with her wondering how her Sempai would be if she were still alive.
Also...being sold? Why would a rich family need to sell their only daughter? They definitely didn't seem like they were falling on hard times.
As for the "no other dimension" stuff...that surprises me in a story that trying to be a bit more scientific...as even in RL, scientist have accepted the possibility that countless alternate dimensions exist. "String Theory", I think?