My money is on Justin for Malcom's murder, but that's just a WAG. I don't think he was anything other than a stage magician.
Justin has my vote, too. And Malcolm doesn't have to be anything more than a stage magician for it to make sense.
Killing Malcolm frees up Harry and puts him in the system, where Justin can easily keep track of him and wait. If and when Harry shows any magical talent, Justin can swoop in and "rescue" him -- Harry's years of loneliness and hurt in the system, where he's been
craving familial love and a father figure, would make him ripe for Justin to manipulate.
We see that in Ghost Story -- the couple times that Justin actually acts like a father toward Harry mean
the world to him.
I would give it good odds that Elaine's parents died in similarly mysterious circumstances.