If I'd have to critique the story honestly, I'd say it's too short and too much like a children's story right now. Unless it will get a bit of a campy vibe to alleviate the unrealistic parts of the story, this story won't work well. The protagonist doesn't seem talented in investigating but neither does anyone else in the story. In fact, the MC seems like an adult child with no concept of consequence like the ones you see in children's stories.
From a more adult and realistic perspective though, this story wouldn't do. Even if the police were just following protocol, they would've made the link between the car accident and the first murder, or at least between the first two murders. In either case Camron would've been put under early arrest as a potential suspect. That Avril made the link between the murders and suicides around the end didn't strike me as smart at all, anyone could and should have figured it out sooner. Even if there weren't very obvious clues left in the crime scene (and yet Avril follows longshot leads like coal delivery and pastries?), the whole investigation part of this story seems like the simplified version that one sees in children's stories. So, unless the story is adapted to fit that PG rating by writing away the blood and corpses, I highly doubt this tale will have an audience that will really like it.
Avril's character itself and the beginning of the story felt rushed too. It felt like Avril's character development was being fleshed out in the first page, and the rest of the agency in the next three or four. Avril's clumsiness felt added on rather than being a trait of hers, and in this case I would actually prefer a blank slade over a clumsy MC like her because her personality actively disagreed with her having any investigation talent. Rather, it painted a character who would unintentionally tamper or destroy evidence, and I'm pretty sure she even did so at a few occassions. Even if she wouldn't sneak into a crime scene and steal evidence, which, by the way, she should've been arrested for right away. She should've been the main suspect after the first investigation because she was stealing from a crime scene and had no investigator's licence to back her up. She was literally carrying a bloody knife murderweapon when she first met O.
With other characters, be careful with assuming that something is a trait of theirs in the reader's eye. F.e. Camron's habit of calling Avril by a wrong name was only used once so the joke that he was right and she corrected him wasn't amusing to me. I'd either eliminate such moments relying on character development, or expand upon the character development scenes a lot more to make them work.
The part of the murderer's identity, I'll leave that up to other reviewers to determine whether it was obvious. I already knew who did it about a page after their character was introduced, but then again I have that with a lot of stories. It's one of the downsides of short detective stories, there's only a small cast amongst whom the culprit is hiding. It's just like a CSI or NCIS episode where you know that the one-episode hollywood-looks actor playing a not-jerk is the 'surprise' culprit.
It was pretty easy for me to determine early on that Eileen was the culprit all along, with her love for Camron and the murders obviously being in his favour but with him being too obvious as a culprit.
I don't know what your plan with the sprites is, but if there's not a big budget or artist motivation for them I'd suggest scrapping a few. There are a few characters who seem arbitrary, such as most of the young versions of the characters. Quite a few of them were not really necessary or adding much, if you want to keep Avril a defined character then perhaps try replacing the other characters' backstories with more of hers to paint a better picture. Right now, the orphanage story and her current life don't seem too connected.
Final conclusion: As you may have guessed I wasn't much of a fan. I'm not saying it won't work, but I think you should make some hard decisions on what you'd like to do. I'd say that making it more like a children's story and eliminating all the gore is your best bet, really make it so that it becomes abundantly clear that this is not a story for realistic and sceptical jerks like me. Making it campy can work too, but doing that right is always difficult.
If you want to keep it realistic, I'd suggest you expand the script a lot with more character development, make Avril more capable and skilled as an investigator with some talent or wit of hers, and add consequence to the story.