Building off of your #3) An example for this would be the botting programs made for Runescape. Developers wrote an application that would sit on top of the game client and then use reflection in Java to see what's happening in the game: Various objects being created that are now on screen, the players direction, the map grid, etc.

The developers of this botting program would then expose their own API and scripting language that would allow other developers to interact with the game through their program. The could then write scripts such as: Go to this location via map coordinates, pick up some object, solve a puzzle, fight some boss, eat when below a certain health, etc.

Note: I used to run a VPS company that would specifically cater to those that were botting in Runescape; this allowed them to run their bots 24/7 without using their own hardware.