Question about Solivagus ruins SotF Spoilers

!Confused about this chapter from SotF, looking for a better interpretation than my own. Veridius tells Vis and Co that they got Belli's body back by using the ruins to sacrifice Marcus Carcius which creates a bypass to the Labyrinth and allowed them to go in there and get her. From the wording it seemed like they almost had to give a body to get a body, was that the case? From what I recall they also mentioned they think these ruins were built to bypass the Labyrinth.

!If they sacrificed Marcus Carcius to bypass the Labyrinth to get Belli, why didn't Veridius sacrifice other people who were willing to guarantee someone becoming synchronous? Seemed like he was sacrificing kids for ~7 years anyway with no success, figuring utilizing this bypass would have been used earlier by him?