How is she acting creepy? Sure, holding on to an unrequited crush for that long is kinda weird but she never tried to take advantage of Koto and her decision so far have been remarkably level-headed for someone with so much emotional involvement in the situation.
Whether she was really responsible for the disappearance isn't completely clear. Even so, I'd say it's reasonable for a teenager to not expect a random "I wish she'd disappear" to suddenly become reality.
This.
In chapter 3, Erika was making sure Koto talked to Aya obviously for no good intentions.
"You have to make it clear to her if your feelings died down."
Koto never said she moved on.
Erika's action here clearly was just trying to manipulate Koto to the direction she always wants to be when Koto seemed to haven't figured that out yet.
After all, Erika makes it clear in chapter 2 she knows Koto's feelings for Aya are just as strong as ever.
Erika only wants Aya back cauze Koto can't move on.
She is really creepy for hoping their relationship to crumble so she can finally grab a chance.
Koto and Aya are her friends, not her enemies.
Erika says it herself, she's really awful and disqualified to be their friend.