So heres the deal,
Mew is not obtainable in the japanese version either. Shigeki Morimoto, one of the games programmers, designed and created mew and put him in the game without anyone knowing. The only legitimate way to get mew in came is faraway island, otherwise its cheats and events. You can also get him through a glitch in which the games programming messses up pokemon trainer 151 with wild pokemon 151 and mew is in the grass.
Missingno was not a removed pokemon. pokemon in the game are labled from 000-151 Well guess who pokemon 000 is? thats right, missingno.