1. Jagex made it an NPC in the game code, ask them why.
2. Mixture of the one that's easiest to see with the eyes at a glance, and the one closest to the mouse.
3. Either have it spam the issue in the logger, or stop the script and give the reason the script stopped. Most normal users won't know what exceptions are, and not really a reason for it here.
4. Probably mean this I guess