So I could be wrong (but I really wish I was)... IF Arika WAS doing drugs, then the whole cover-up wasn't just to hide her murder, but also to prevent any investigations from happening at all. Hiding her body so it couldn't be analyzed for traces of drugs in her system.
If I remember right, she was one of the most popular in the group at this time. An idol being caught smoking or having a boyfriend is one thing, but an idol doing (possibly) illegal drugs? Something that big isn't just gonna blow back on Arika, but everybody in the group; ruining all their chances.
Again, hope I'm wrong. This could all just be some set-up Emily made to get Nana to give up without resorting to whatever Chihiro is doing.