There are many Englishes, and each of its dialects is valid for communication among its users.
Standard written English is not the One True English; it is a dialect that is useful in some, but not all, contexts.
Language snobbery is not more noble than other forms of snobbery. When someone writing about grammar and usage begins to use terms like “illiterate,” “hoi polloi,” “the masses,” just stop reading.
It is not your fault that you were taught bogus rules of usage. You can unlearn them.
Use or do not use the Oxford comma, as your taste or house style determines. And don’t make a fuss about it.
To determine a point of standard usage, consult Bryan Garner’s Garner’s Modern English Usage (fifth edition), Merriam-Webster’s Dictionary of English Usage, Fowler’s Dictionary of Modern English Usage (edited by Jeremy Butterfield), and Benjamin Dreyer’s Dreyer’s English. Preferably all four. When they don’t agree, you get to make up your own mind.
Five books that can enlarge your sense of the language:
Robert Lane Greene, You Are What You Speak
Henry Hitchings, The Secret Life of Words
Gretchen McCulloch, Because Internet
Jack Lynch, The Lexicographer’s Dilemma
David Shariatmadari, Don’t Believe a Word
Language is the most democratic thing we have. You get one vote.