For the purposes of this episode, the light switch in Harry's office was moved to the left side of the door, in later episodes it is returned to the right side of the door and the wall can be seen to be patched where it was.
Mr. Slotkin holds the courtroom hostage with a gun taken from the evidence table. No weapon is ever entered into evidence while still loaded, and a loaded weapon would certainly never be brought into the courtroom as evidence.
The gun Mr. Slotkin uses to hold everyone at bay is an automatic, which requires that the slide be pulled back to initially cock it. He never does, so it would be relatively easy to disarm him as the gun would not fire.