His wildest choice to me is using “could of” etc. I get it’s for pronunciation purposes, but now that it seems on its way to replace the correct spelling, it really jarred me.
Wild? Using it in dialogue seems perfectly reasonable. It's a common malapropism.
Even in dialogue I would've expected "could've". 'Could of' reads more like a typo
It's phonetically identical or very near it in most dialects. If anything it's a transcription error, but in this case just a way to imply the spoken cadence of a certain accent. Not any more incorrect than spelling "going to" as "gonna" in dialog.
Very often it's not a malapropism, just a variation on pronounciation.
It's not even that, they're pronounced the same in most dialects. It's either a transcription error or an atypical spelling depending on the intent of the author. It doesn't represent a different meaning or pronunciation in speech.