this manga share the same universe with "Mayonaka X Giten" (same mangaka)
Not only that, actualy Saaya is the very first one who created the the body swap method in Mayonaka X Giten
How Saaya creates it?? Probably with similiar trick with the baldy Kuroneko (check chapter 3)
Why Saaya creates it?? To save Minamida Azuma. Minamida simply swaping into better body and he will be immortal
Ofcourse one of the trait of body swap in this universe is super power. Probably Minamida get a power to silence Saaya.
Probably something like "global memory erasure" so change nothing in the rule of Sayaa-cannot-lie
Now, body swap is a cheat power in this universe law because shinigami also count body as sinner
Remember that Kureha-Shiroha arc? If the possessed innocent kid kill the father, she will also be blamed for murder
What if someone (let's say Aida), swap body with another one (let's say X), and kill alot of people (let's say Shizuka Family)??
If we open up the possibilty of body swapping, we got alot of new scenario and explanation of confusing thing.
Combine body swap with "global memory erasure", it explain about the last wish contradiction for Aida's case.
And since body swapping also grant the user super power, It explain why Minamida jr so keen for Aida's body (check last chapter)