My first suggestion would be not to fly indoors. Moreover this module has issues with flying indoors due to lighting and in some cases flooring as it is. Developers themselves state that this sensor isnt apt for indoor flight. If you had read the docs etc or the links posted earlier then you would find that all your questions are answered by the developers themselves, who are at the 'ground zero' if I may say.
The first link previously posted about setup answers your ques 2.
As you can read in arducopter wiki, its not plug and play; moreover nothing related to APM is plug and play at all!
You would have to rewire it , properly connect it and solder and cut some pads underneath the FC. You have to check it by loading a test sketch via arduino IDE or MP and then calibrate the lens by capturing an image by connecting the senor to PC via python Then I believe you can change parameters on how do you want to use that sensor in case of a gps fail safe scenario. I believe you can instruct the autopilot on how much to use data from that sensor etc. And all APM's from 2.0 to 2.8 are almost similar and since yours is the latest one it will work. Connections may/may-not have changed so you would have to look at the schematic of your particular FC and match it with the instructions.
The second link is a discussion where the arducopter dev Randy himself is discussing about the sensor and how it works and stuff, where you would find answers to question 3/4 and even 2. You can even ask questions there if you want so that you can get answers straight from ground zero