Oxford A-Z of English Usage 1st Edition (Android) 2.2

Is it correct to pronounce "mischievous" as a three-syllable word (miss -chi-vuhss) or a four -syllable word (miss-chee-vi-uhss)? If it is right to use "alumni" as the plural of "alumnus," why is it wrong to use "octopi" as the plural of "octopus"? Should we write "12 am" or "12 a.m