General comment about approved trivia, goofs, other information

I see many trivia items that start with "In the books, this character did..." There should be no comma after "In the books" because it is an incomplete sentence, a comma shouldn't be used to separate a sentence unless both clauses can stand on their own as a complete sentence.

"In the books" is not a clause; it is a phrase. In this case I would agree that the comma should not be there, but not for the reason you stated. In this case it is a restrictive phrase, i.e. it is necessary for the sentence to make sense.
In fact, if two main clauses do not have a coordinating conjunction, they need to be separated with a semi-colon, not a comma.

Also if a trivia item is relating to a specific event it should be "In the book" (singular) because obviously an event usually only happens once.

Possibly, but not necessarily. If a film (or series of films)  is made from a series of books (e.g. Harry Potter), using the plural is a way of taking the series as a whole, and emphasizing the distinction between film and text. Also, if there is a negative, (In the books he did not...) the plural indicates that this refers to the whole series rather than the specific volume, i.e. at no time anywhere in the series did he....

 Sentence starting without needed functional words: "Last film in the series" versus "This is the last film in the series" or just "The last film in the series".

If you are going to allow the sentence fragment, there is no reason why the definite article should be required.  This can be considered as the ''headline version', and headlines often omit articles.

When referring to a decade there is no apostrophe (e.g. Titanic is the highest grossing film of the 1990s). An apostrophe indicates possession, e.g. Home Alone was 1990's highest grossing film.

This is the most common current style, but it was not always this way. When I was in school, I was taught that the apostrophe was used for the plural decade and for single letters (1970's, '90's, A's). The current style is to omit the apostrophe except in the case of lower-case single letters ("Mind your p's and q's."), but it is still used by some.

"In the books" can be considered an adjunct clause, but really debating what is a clause versus a phrase in this context is splitting hairs since because my comment was about incorrect punctuation and excessive commas. Though yes, "books" versus "books" varies by situation, e.g. it doesn't make sense to say "In the books, it takes multiple votes to choose Jon as the Lord Commander" when referring to an event that happens in one specific book.

The decade note comes from both the APA and AP style guides.

Trivia and goof items are informal writing (they are headed "fun stuff"), and appealing to style guides such as APA and AP is out of place. I agree that many trivia/goof items are poorly written, but I would not apply style guides. Rather, the criterion I apply is that the items should be clear and easy to read.

I agree with David's comments, and won't repeat them.

Spelling out numbers: even appealing to style, my distant memory from school is that whether you spell it out or use numerals depends on context. Again, this is informal. Not something I'd ever sweat.

Dates: when speaking, we say "July third twenty fifteen", not "July three twenty fifteen". Since this is informal, I accept putting the spoken version into written form.

Ampersands: what's "regular" writing? If you mean formal writing, then I agree, but this is not formal writing. Often it's better to type the extra keystroke for "and", and save "&" for contexts where it's actually preferable, but I certainly won't waste a second correcting them.

Sentence fragments: "last film in the series" communicates a complete thought very clearly and compactly. I think it's fine for informal writing.

Partial thoughts: I agree that this is sometimes a problem, and see it mainly in goofs. Things like "J Harrison in scene 7", with no indication of what's of issue or interest. So delete them -- I've deleted many meaningless goof entries. There's also the issue of continuity "goof" items which no one ever notices unless they are sitting there studying the film for the purpose of finding things they can submit to IMDb ... but that's an issue of "is it interesting to anyone but the submitter", not writing, so I typically just ignore the continuity entries.

All in all ... lighten up and enjoy it. Pick your battles wisely.

Edward