We can't critique here, but there are a several elements in your idea that have potential from a storytelling sense.
Non-linear is a type of mystery
There are many types of mystery stories – people tend to think of Agatha Christie who-dunnits where the reader is competing against a brilliant in-story detective, solving a puzzle with clues as to which character is the murderer. The story wraps up in one astonishing final reveal which the detective (and author) has withheld or obscured from the reader.
Your story suggests more of an American-style criminal investigation that is pursued: one witness leading to another, which leads to another. The nature of the 'crime', as well as the reader's mental construct of the criminal, grow over the course of the story – not so much twists and misdirection, more about uncovering new layers that add depth and scope to the crime.
Courtroom mysteries also tend to take this form, where each witness tells their part of the events, filtered through their biased testimonies. This is kind of a hybrid of the other 2 types of mystery. One witness may be lying, but just as often the 'mystery' is about resolving seemingly contradictory statements in a way where the contradictory testimonies are actually true.
the witnesses are... dead?
It's not clear if this is your intent, or if I am making assumptions. The idea is more interesting (to me) if the witnesses are the dead victims of the MacGuffin monster.
Sonya is not just a wizard, she's a necromancer who is experiencing(?), channeling(?) the final memories of the dead victims. Throw in a bit of lore and worldbuilding: she might be a Witcher-type of roving samurai that is not especially liked or welcomed by the villagers.
The particular rules of this paranormal ability can be engineered to create a built-in, on-going conflict which helps drive the plot. The method might involve access to the victims' remains, which might be in various states of decay and memory degradation.
Her ability may be taboo, and kept secret as long as she can. The victims' families might object to the sacrilege, and re-victimization of their loved-ones. There might be mistrust and revulsion, making her job harder. It might have a taxing effect on her physically and mentally (an effect you will need to show, not tell, for it to add to the stakes).
Protagonist and reader connection
Another aspect of your idea that has potential for great storytelling is the reader and protagonist can be kept closely in-sync. The reader learns as Sonya learns. Sonya jumps to the same (wrong) conclusions as the reader, based on the same information as it comes.
Sonya is an outsider. Her observations are the reader's observations. Her discoveries are the reader's discoveries. No need to hide clues from the reader, or keep Sonya tight-lipped about an early clue that solves the whole case. Reader and protagonist can remain on the same page until the end.
Her POV might be exactly the same as the reader's. Maybe she is forced to trust an uncertain ally because she is vulnerable while doing it. Upon awakening, time has passed and she will need to catch up with events that can be explained to her. Trust me, head-hopping does not solve all narrative problems, you will still have scenes where someone just needs to explain something to the MC.
The narrative includes these other people whose stories are all varied – being the memories of their final day, but we keep coming back to Sonya who has growing multiple (sympathetic/antagonistic) perspectives of the same villagers through differing POV.
Why she is our MC
She may be investigating in an official capacity, in which I'd expect her side-conflict to involve politics and local resistance.
If she is pursuing for personal reasons, I'd expect some backstory involving this monster and her unhealed trauma, something that is preventing her from settling this case.
If she is a reluctant or accidental detective, the side conflict may be about trying to convince people to believe her, or staying out of the official investigators' way.
A good story will play with the MC being both the perfect person for the role, and also the last person who should be in that role, giving the MC a dichotomy with rough edges that get in their own way.
At some point the side-conflict will overtake the 'main' conflict of a monster eating villagers, threatening to derail the investigation and undo everything she's accomplished. This keeps her character grounded in the immediate conflict of real people with their real (not always honest) motivations.
Maybe I am reading in too much
Maybe I am wrong and Sonya is simply a bridge character, and there is no supernatural purpose to the head-hopping, it's just an ensemble story with multiple POV. If that's the case, there is no structural 'gimmick' to the idea. It follows typical TV show omniscient camera.
There is no narrative need to have Sonya 'dream' their side-stories. The narrative voice can follow any character as long as it's convenient, using flashbacks and non-linear time skips.
In this case I do not see much purpose to keeping Ether's identity a mystery from the reader. It almost sounds like the cliché in every cozy-murder TV show where someone opens the door and says "Oh, it's YOU!" and then gets murdered in a way where the killer is obscured behind the door. Aaaand commercial.
This doesn't have the same effect on the written page. Assuming there is something special about Ether, they shapeshift or are somehow perceived differently by each victim, there is no 'mystery' about keeping an obvious identity from the reader (obvious to the person who opened the door and got killed), when our MC is literally reliving their memories.
Readers don't like fake-mysteries, or things that act like mysteries for no reason other than to prolong the story.
There are certain promises made to the reader when you present your story as a mystery. It doesn't need to be a who-dunnit, but the 'mystery' does need to evolve as the story progresses.
