I think you get into the "dark arts" when it comes to "the number of m's in 'recommend'" or whether DJ'ing needs an apostrophe, or in dates eg the 1940s (1940's).
If it's DJ'ing, then why not D'J'ing, or, for that matter, D.J.ing?
For the apostrophe, a succinct flow diagram could be written for normal use, as in possessives, both singular and plural, and where a letter is missing. To my mind, where two words (disc jockey, member of parliament) have been abbreviated to their initials, then a full stop represents the removal of multiple letters, the apostrophe just the one.
Is there an accepted set of rules for these anomalies, or do we just muddle through?