* 1 The non-ASCII apostrophe is always ’, so programs that automatically insert smart quotes do the wrong thing for apostrophes at the start of a word. Some better examples IMHO: ’tis (it is), ’nuff (enough), ’cause (because), ’em (them), ’fraid (afraid), ’n’ (and),
* 2 The "name ends with s" rule also applies to names ending with "x".
* 3 is arguably just a special case of 2 (e.g. we can say "a holiday of two weeks"), but note that this is very similar to the attributive use (7) which never† uses the plural or an apostrophe but does use a hyphen (a two-week holiday).
* 5 There are, in fact, grammatically-valid uses of "its’", when "it" is a noun rather than a pronoun. A non-derogatory example, in some variants of tag (the children's game), the its’ advantage increases over time.
* 6 Whether apostrophes are used for plurals of non-words is a matter of style, not correctness.
* 7, † Some apparent exceptions are due to words that are actually collective instead of plural for the particular use case. For example, "arms industry" is like "clothes industry". More often, people use errors and omit the required apostrophe. "farmers market" is an abomination.