Why did Modern Tamil reduce its alphabet from Middle Tamil to Old Tamil?

Question

It is generally known that Modern Tamil (since around a century) totally simplified its alphabet from covering all Indic consonants to only consonants in Old Tamil (as written in Tolkāppiyam grammar using Vaṭṭeḻuttŭ, a descendant of Tamil-Brahmi).

Even Middle-Tamil had coverage for all Indic consonants, which evolved from Pallava script to the Grantha script.

So when exactly did this reform to remove non-South-Dravidian consonants from the Tamil script happen, thus standardizing the present day modern-Tamil script?
What is the reason and history behind that? (I heard it's mostly because of anti-IndoAryanism)

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I found this line on Wiki:

The Tamil purist movement of the colonial era sought to purge the Grantha script from use and use the Tamil script exclusively. According to Kailasapathy, this was a part of Tamil nationalism and amounted to regional ethnic chauvinism.^[13]

What does it mean to remove Grantha script and use only Tamil script? Isn't Tamil script already a descendant of the Pallava-Grantha script?

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At a later period, Modern-Tamil standard seems to have allowed a few additional consonants {ஜ,ஷ,ஸ,ஹ} in its alphabet. When did this exactly happen? And what reason? Is it to reproduce sounds in texts like Bible?

And why are these called Grantha consonants? Isn't the entire Modern-Tamil alphabet a minimalist Grantha script?

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Apologies for too many questions. I believe a proper brief on the history & politics of Tamil's linguistic-evolution would clarify all the above queries. If possible, kindly also provide references; it would be immensely helpful. Thanks!

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Edit:
It looks like there were attempts to include the full Tamil-Grantha script into Tamil Unicode by the Indian government, but it seems to be not approved by Tamil Nadu government, calling Grantha as a script for Sanskrit.

Do they implicitly mean that Tamil script derived only from Pallava script, which is different from Pallava-Grantha script; the former having only Old-Tamil consonants and the latter having full Indic-coverage? If so, are there any references to show Pallava script only has the minimal-Tamil alphabet?

Answer 1

The article The Tamil Purist Movement by Kailasapathy details the development of this movement, and pins the movement to 1915 when the movement was officially launched by Vedachalam. You can call the motivation anti-Indo-Aryanism, or pro-Tamilism, but the former seems to be a more accurate term. What it means to "remove" letters is to "no longer sanction their use". The English alphabet used to include ƿ, ð, þ, æ but they have been removed. Even so, æ is still used in old-fashioned spellings like "mediæval". The Russian alphabet officially eliminated some letters like ѣ and I in 1917. Removing Grantha consonants means removing those consonants from Grantha which were introduced in order to write Sanskrit and Tamil in a single script. Presumably allowances were made for s ஸ because /s/ is now a begrudging phoneme of Tamil, whereas [pʰ bʰ b] are seen as fully non-native. See this overview of Tamil phonology for discussion of marginal phonemes in the modern language.

I think the idea is remove all of the letters added in the Pallava script (and later developments) which were not in the Tamil Brahmi of the Tolkāppiyam. The Pallava script covers all of the phonemes of Sanskrit, and does not include the distinction த் t̪ vs. ற் t, so Pallava script is both inadequate and superfluous for Tamil.