1. Usually RNG, but there are items/miscs that have a predetermined drop rate (like 0.001%, 50%, etc.). I would suggest farming in a private room, as you won't have to wait for them to respawn because someone killed them.
2. In Bot Manager (in Le Bot for example) you have there the "Cell Pad" section, with a "!" button, which determines for you the room coordinates. It's kinda hard for you to determine on your own the coordinates, so I suggest using the "!" button.