Here's what I believe:
Language is how we communicate. It's not a SPAG lesson. There's nothing fundamentally correct or incorrect about any of it, other than what we collectively agree to. No-one has any authority to impose their vision of language on anyone else. Language is democratic. The "rules of language" aren't like the rules of Scrabble. No-one invented them before the game can be played. They are just an attempt to codify what already existed - i.e. how we already communicate.
Yes, written language is more codified than spoken language. We largely agree on the correct spellings of words. Cat isn't spelled "katt" but, actually, it could be in a text or an ad or a poem and we'd still understand it in context.
"Alot" is not the standard way of spelling "a lot" until enough people agree that it is. Yes, it's clearly "wrong" in that it's generally accepted to be incorrect, but that is not a fixed position based on any logic. It's just current best practice.
The "up" suffix is something different in "work it up", "change it up" (I've even seen "Google it up"). It's a choice, not an error. I guess it's an attempt to be more emphatic or positive, but it's not really for me to say.
Why might someone invent a new word use when they don't need to?
Because it sounds fun, because it sounds interesting, because it rhymes, just because.
We play with language all the time, some of us more than others. (A lot of new English language comes from AAVE (African American Vernacular English) and young women are more likely to create new usage than young men.)
There are loads of ways of talking and writing that I wouldn't use and don't love. They're not wrong, though, just like the music I don't like isn't wrong, or the food I don't care for isn't wrong or the accents I don't speak in aren't wrong.