I don't see the 22/2 reinitiating a case of GAS. It is a nice lens, but, at least for me, very much a little used complimentary piece. I prefer the 18-55 on my M. The logic is simple, when I want top IQ or low light abilities, it will be with the 5diii. I use the M due to its size and convenience. I find the zoom more convenient. I use the 22/2 when I need the smallest size possible or when I am using the 5d with the 100-400L and want something small and wide. For example, if I am shooting birds while kayaking I might have the 22/2 on my M in a pocket in case I want a wide angle shot.

For Disney, it depends how small you want to go. Personally I'd favor the 18-55. But if you want the M in a large pocket I could see the 22/2.