"She has no reason to trust someone from the world of officialdom. His relationship with those authorities is a bit odd, but still. Third, the mere fact that he's making really big promises is a strike against him--everyone she's ever met who has talked big about how good they will be to her has been lying with evil intentions.
(In actual fact I think his intentions are good, but he's so in love with seeming clever that it undermines his attempts to seem like his intentions are good)"
well said. Sachi has major trust issues,as can be expected. She doesn't even full trust Mister,and continues to view the detectives as a likely threat.
There is also the fact that she has become attached to Mister, though she probably does not recognize this. He was the one who saved her when she wanted to end everything,that too twice, and that is not something you can easily forget
But she can be quite rational, and probably understands that this method has less risks for her,if it is successful