RealCiels soul was taken years ago. Sebastian is OP but not that OP to be able to hod on another person's memories.but to only obtain their power.
RealCiel's soul might be taken by Undertakers at the year he died and kept the memories (Film) of RealCiel, resulting in him getting fired because he violated the rules of not placing the memories in the official and distinct place which was meant to be stored.
Yet he still took his Death scythe without anyone realising it. If he can take his own death scythe which was meant to be taken away by the reapers undetected, it may be possible that he can restore someones life without any complications in their body whatsoever.
And thus resorting to RealCiel being alive and has consciousness throughout his body.
Also, there was a chapter when Undertaker took blood from "Ciel"; that could be conndcted why Ciel is back in town.
We theorist are fucking geniuses ╮(￣▽￣)╭
Remember when sebastian was stabbed by undertaker? I dont know if this makes sense but maybe MAYBE when undertaker stabs Sebastian and while his past were played realCiel's soul was taken by undertaker ??? What do you think?