I think that 'could of' comes about from 'could have' being abbreviated quite legitimately to 'could've', which sounds the same as an ellided 'could of' = '/kʊdv/'

But it may then be expanded out again for emphasis, into 'Well I could of done it if I'd wanted'.

Is that ignorance of or deliberate transgression of the grammatical structure, I wonder.

I understand that 'finna' comes from 'I'm fixing to'. It does seem a trifle unnecessary. Are we really in such a hurry to enunciate our thoughts?