Why YSK?
The first person who typed “should of” probably heard of it in real life that was meant to be “should’ve”, they typed “should of” online and readers thought that it’s grammatically correct to say “should of” which is in fact wrong and it became widespread throughout the years on Reddit.
I hope something could start to change.
Isn’t “have” either an auxiliary verb or verb and “of” a preposition?
Yes.
Are these acceptable? If yes, why? If not, why not?
No, because you constructed them by merely replacing the verb “have” by the preposition “of” in situations which have nothing to do with “of” after “should”/“would”/“could”. I’m not sure what point you’re trying to make, since neither I nor the people I cited ever claimed that this should work in the first place. The claim of in particular the author of the first paper I cited is that for some speakers there seems to be a novel construction modal verb + “of” + past participle, not that the preposition “of” has the same function as “have” in this case or in any other (in this case, the novel construction as a whole would have more or less, but not entirely the same function as modal verb + “have” + past participle, but “of” would still be just a preposition).
I don’t know man, Oxford Dictionary (click Grammar Point to expand) says that […] it is definitely considered wrong in standard English.
Yes, it certainly is considered wrong in standard English, but the interesting thing is that in some non-standard dialects there might be genuinely a novel grammatical construction which actually uses the preposition “of”. I mean, you don’t need to find that interesting, but I do. And if that is indeed the case, it would mean that the speakers of those dialects are not making a purely orthographic mistake like when people confuse “they’re” and “their”, for example, but are rather speaking or typing in their dialect.
