If Chanel is the killer that explains why Chanel nº5 was attacked by the RD several times but always survived: the RD has no intention of killing her, just bully her. Just like Chanel always does!
Chanel killed Rodger and Dodger to punish Chanel n5. She killed Sam because she betrayed Chanel N3's trust. She (or Boone) hid in Chanel n5 car and killed the truck driver to scare her back to the house. Plus the RD never attacked Chad. And she killed Chanel n2 after she said she wanted to leave.
That person said he didn't watch the Dorkus episode, just The Final Girls one so maybe those things are clarified in that episode. Who knows really but I would love that ending.