I would have silently gone in my head ABCDEFGHIJKL and then said "MNOPQRSTUVWXYZ", but I suppose I could have said "MABCDEFGHIJKLMNOPQRSTUVWXYZ", or should I say "MABCDEFGHIJKLOPQRSTUVWXYZ"?
Come to think of it, why do our letters have a standard order, and if they have to, why that one? A better order, off the top of my head, is OUEIAPKGHXBDPTMNLWYVZSFRCQJ
The reason the alphabetical order is like that is because it was passed down all the way from the 14th century BC and potentially before, so a lot of sound changes and mild order changes happened (like Z going from the letter after E to the last letter) and you have the modern English alphabetical order
Alphabet starting with M? Hey, prescriptivist dingdongs, you can't just say there is now apparently an alphabet that starts with M! Those things don't exist in vacuum - if you make something like this up, you have to go all the way and define such an alphabet clearly!
Rules mean nothing unless they clarify established language use conventions! Which is why I think descriptivist approach is better in most cases.