SPOILER/THEORY AHEAD
what if the mastermind is Dave?
like the Dave from the a previous timeline
that would explain a few things
if we asume that Thanatos is reciving its orders from someone in a previus timeline then that means that the mastermind in this timeline might be just as clueless as everyone else
this also would explain what happen to Sal in day 10, whoever was giving orders to Sal would need to have a lot of imformation that couldn't be obtain on the spot, they need to know that Sal is vulnerable to hypnosis and that he could be leaded to kill someone, if the mastermind is from a previous timeline they could know all of that and order Thanatos to give commands to Sal
Florencia said that Dave could also a mastermind as his choices determined much of what happened, this could be foreshadowing
i know is kinda crazy but i still wanted to put this theory out there