I don't think that to be the case. The creators made it a point to show that Jo wasn't in the school and was arriving. I can see two reasons for that, either we are meant to think that Jo isn't coming and had no intention to do so, or that she wasn't there and had no power to do anything.
I believe that if Jo had never arrived as she said to the MC he would've mentioned it - although this could be seen as her showing up just late enough to have plausible deniability.
Now I'm inclined to think the latter option to be correct, as it's a point made during the mission that staff wouldn't be in attendance of the party, so it'd be the perfect chance for the person behind the doll to do their thing without Ms. L, Jo, or possibly the janitor interfering.
As I'm writing this I'm sorta starting to suspect Isabelle, as she:
-Is known to be vengeful, taking things too far in her pursuit of justice and Lindsey stole her precious chocolate.
-Wasn't in the school before so she's an anomaly.
-Came up with the party idea, giving the doll the perfect chance to strike.
Maybe Maya is a red herring, as it'd be too obvious in my opinion.
I wouldn't put it past Jo to fake arriving, in fact that's what I expect that type of duplicitous character to do.
With that night being so crucial to the plot, I feel like it's safe to assume everyone was at that school in some fashion. So if someone is unaccounted, they're automatically suspicious (hence maya). Whereas Jo may not have been right there secretly supervising the events of the party, I highly doubt she was sitting at home reading a book by the fireplace as her foil was doing covert stuff at the school. If her foil was there, it's safe to say she was too. So if she is Mer, it's really more just a matter of what exactly she was doing there imo.
I actually entertained the idea that Maya may be a red herring, but ultimately I ruled it out for two reasons.
- Between the bullying and rough home life, Maya's storyline lays the victim narrative on super super thick. All her quests so far revolves around witchcraft and her dark arts cult background. And yet conveniently, she is cleared from suspicion because she is portrayed as the victim of all of this. That is the perfect cover for a villain as it gives foreshadowing but doesn't give her away.
- The AI is written as a character that is both all seeing and not swayed by emotions. So the way it could be fooled would generally be misinterpreting human emotion. However here the opposite is what is needed. It is in fact the perfect tool to see past emotional manipulation and lies that would trick most people. The AI also doesn't make for a good red herring since it had no other role in the story to give it credibility. It's basically banking it's own reputation as a character on being correct with this assertion.
While the isabella take is interesting, her events are too in focus and dynamic throughout that night to really be involved in a second conspiracy imo. It is possible, but that character would be doing a little too much on that night between hosting a party, organizing a trap against kate, cleaning up and/or busting MC.
Furthermore, she may not even want vengeance depending on what route you picked with her. I see her as more of one of the target girls for Mer's manipulation. And if any of targets have some greater role in the shadowy dealings, I'd say it's probably Kate since Jo seems most obsessed with her.
