Common Candy is a good theory, but no, the actual method is actually quite genius, kudos to you Ame (I've said that on another thread already but hey, double kudos? )
To evolve Inkay, you have to try stopping it's evolution WHILE its evolving, in other words, try and cancel it. By doing that, you make it a surefire evolution to Malamar.
EDIT: I double checked on this by cancelling my Sentret's evolution to Furret once and sure enough, it stopped evolving, but when evolving Inkay (the first Inkay I did I accidentally got it to evolve) I cancelled it because I wanted it to learn a move early on. But then the cancel failed and it became a malamar (: