gator bhayya, I'll try answer in the same order as your questions.
As for Baba Farid or Waris Shah or any other Punjabi stalwarts using 'nay' while expressing intent - I'm not even aware of any of their work in Urdu, and its a shame I know too little Punjabi to really appreciate their Punjabi work. Will work on that lacuna (can you help?) once I'm done learning some Hindi vocab., which in itself is pretty interesting.
I'm not a linguist and not familiar with all the terms. What does 'shift grammaticalization' mean? If you mean a change in a grammar rule along the chronological line, then yes, it has become widespread during the last 50 something years. Our parents friends don't make this 'mistake' if you will, but mine do.
Over the years in Pakistan, Urdu has become more and more Punjabized. This is because of Panjabis' overwhelming acceptence of Urdu. But they've invented their own 'dabistan' ('maktaba-e-fikr' or school of thought) instead of adhering to either to that of Delhi or Lucknow. That should already tell you that most Panjabis in Pakistan use 'maiN nay' instead of 'mujhay'. But so have others more likely to mix with them, congregate together and intermarry, notably Pukhtuns and Muhajirs. Then its all the time on the tele and in the papers, so naturally the Pakistani dabistan-e-Urdu has taken that turn, we like it or not. That doesn't make 'maiN nay' grammatically right though! Only popular, but not correct. Like 'mashkoor' in lieu of 'shaakir' or 'shukr guzaar'. But its true that the problem is not having learnt the language properly, the lack of proper schooling is the culprit. Faylasoof miaN gave the example of Iqbal, education made that difference.
'mairay ko' as I said earlier (and I think you missed reading that part) is popular among the Gujaraati community, spread from Karachi to KathiawaR to Mumba'i, and among those Urduvalay who speak the baazaari language of Delhi (I'm NOT talking of the aristocratic khaRa Dehelvi accent). And of course all the youth who can't sleep until they've seen at least one Bwood flick, from where they pick this lingo. I haven't yet heard this outside of the Karachi+its suburbs ka Ilaaqa.
Tashakkur sad baar for your welcome. Biraadar-e-azizi, it might shock you but Purbi language isn't/wasn't spoken in any Purbi region (east India, bengal etc). My great-grandparents are from Lucknow and Kanpur, and the ancient-sounding dialect they spoke was called Purbi('Eastern' in Persian). It was the language that Amir Khusro did some of his poetry in and many qavvalis to this day are in Purbi (eg 'aaj rang hai', 'man kunto maula', 'naina mila'i kay'). For me the distinguishing feature of this dialect is the borrowing of entire phrases and not just words of Farsi and sewing them together with Hindi. I think this dialect is called Brij Bhaasha nowadays. The shift to Urdu began during gramps' generation when they attended Aligarh U and mixed with Urdudaans of disparate backgrounds. Then '47 was a watershed in this transition to a more standardized Urdu. By my generation the link is practically lost, and so is that with Farsi proper, though at a personal level I'm trying to resuscitate them links.
Falsafi bhayya, thanks for rallying for the cause of grammatical correctness. The problem is that Urdu in school books for the Urdu language course may be correctly explained, but the teachers don't speak it right and the parents at home don't speak it right. The poor book isn't even read thoroughly so it can't make a dent in whatever incorrect grammar the spoken word is pouring into kids' brains.
I'm only trying to rectify my own mistakes, those of close friends and those of language lovers on this forum.