A) Well, you don't use a GameShark to go to gamefaqs and it's still cheating...
B) you may or may not say it's cheating: You aren't fiquring the answers on your own, yet the answers are in the game
C) Yeah, it's impossible to not find Luigi unless you're blind, in which case you'd have trouble playing any video game. He shows up right where you disembark from your last mission, like the dock after chapter 5 and the station after chapter 6.