I don't think sebastian's contract will be affected. In the past they created the zombies only with a body and fake memories, so I think that RC! soul is not inside of him. I think undertaker just perfectionate his zombies. Plus Sebastian did say to OC! that he ate RC! soul, and he can't lie to him.
And as for undertake's plans idk he is pretty hard to understand, he was kind of obsessed with Vincent (and maybe Claudia) so maybe he had a plan to erased the burden of being the queen's watchdog (I'm just randomly saying theories)
So many things!! but what if the contract is not real? If he never got full payment he can pick and choose on what to lie about. After all demons are meant to decieve. He probably stuck to the truth most times to drive our ciel into vengeance and cruelty. He just looking for a really delicious meal so loyalty and a demons word is murky water. Also undertaker did something that made him quit being grim reaper or what ever they are called. Something to do with claudia and the phantomhive line but what if he is part of that line?! Remember that to be like grell (i don't remember the name they are called so ima say Reapers! Lol) they had to commit suicide to become one, what if he is related to them? His eyes changed color wen he became a reaper so he might have had blue eyes as well. So many things can happen! But i wont put it past Sebastian to be a crooked butler
I mean we really dont know undertaker age lol he went to school with vincent maybe but Claudia came b4 vincent so how and why would he have her mourning locket?.
Actually I read yesterday the next chapter and they show how Sebastian and OC! talked about the contract and how they discussed the terms of it. OC! made sure Sebastian didnt fool him.
and as for undertaker I don't know hehe, the only measure of time he has, is on the campania arc, when he said to OC! that its being over half a century since somebody call him a Shinigami (that was the name haha). But that doesn't answer anything tbh, only thatmaybe he stopped at the time Claudia was alive (or about to die).
We need answers
Maybe this Real ciel is the goal that undertaker dolls was after? He finally perfected it? He also held our ciel hand to his cheek when he was sleeping so why would he bring back the real ciel? Maybe he doesnt want to see our ciel suffer anymore and out of the clutches of Sebastian. Also does this make Sebastian contract null and void? Since payment wasnt fully transacted if the real ciel is in fact real and not a doll does he expect to be paid? Speaking of him(real ciel) why blame our ciel? He didnt willingly sacrifice him, he just lost faith in god in that nightmare so why blame him? He set on revenge bcuz of that night. He isnt exactly living a happy ass life so that real ciel need to stop acting like a dick.