Hmm...can I make my own custom Mario Kart 8 tracks with this? Furthermore if that is possible how would I go about dumping my copy of the game to an ISO?
I don't think so (now) Maybe in the future but Maybe, beacause you would need another program to edit then put it replacing a track.
Yup, replacing stages works fine. You can do the same with DLC characters. To edit music, they are in audio/stream. You can replace them or even make your own music to use with a converter.
That's Nice