After some research I have found out that the black smudge is actually all the dead pixels. What "killed" them is the LCD being put under tension, not pressure. A drop can cause this. This only happens to AMOLEDs. Hope I helped someone.
You cannot remove the button without removing the glass. Removing the glass is fairly easy, only tools you'll need is a spudger and a heat gun. Tricky part is cleaning the glass or lcd and reattaching them.