Dear happy, you have made an important point. The term "anthaNar" is a much mixed up term. There is not much of conflation with the terms arasar, vaNikar, and Velalar. There definitely was a time that may include the Sangam period, when the term anthanar did not automatically mean Brahmin.
Originally Posted by happyhindu
Take for instance Thiruvalluvar, he uses the term in the phrase அறவாழி அந்தணன் (aRavAzi anthaNan) in the first chapter. Here, there is no doubt it refers to some sort of god or acharya, not Brahmin.
The other place Thiruvalluvar uses this term is in நீத்தார் பெருமை/greatness of renunciates. In the context of this chapter, the natural way to understand the term is to take it as Thiruvalluvar's description of a renunciate. However, later day commentators who were influenced by Brahminism, such as Parimelzagar, did interpret this word to mean Brahmin. But, a neutral observer who looks at where the Kural in question appears and the general tenor of Thirukkural as a whole (e.g. only dharma, artha and kama, no moksham), surely will see it as a travesty to imprison Thiruvalluvar's anthaNar into varna system.
Renunciates being called anthaNar is found in other Sangam literature as well. For example, in Kalithogai #9, anthaNar is described as one who holds thridandam/முக்கோல் and kamaNdalam/கமணடலம். These are what people even today understand as marks of a renunciate, not a Brahmin.
The Sangam literature itself is free of any detailed definition of Varna system like what one will find in Sanskrit texts everywhere, starting from Rg Veda, ithihasas, down to BG, let alone the toxic dharma shasthras. Any unmistakable reference to Brahmins in Sangam literature, like பார்ப்பனன் (pArppanan), is almost always peripheral to the main topic of love or valor. It is hard to really come to any definite conclusion from the sangam literature as to who they were, whether thrivedis from the north, or just atharvana vedis as you suspect. As you say, more serious research is needed.
Later, by the time of Manimekalai, the term was already appropriated by Brahmins to mean just them. By this time it is clear that there was opposition to Brahmins and varna system. Seethalai Sathanar in Manimekalai was quite derisive of Brahmins.
Come down some more in time scale, and we come across the following poem by Auvaiyaar, a law text (நீதி நூல்) called Nalvazi verse #2:
சாதி இரணொழிய வேறில்லை சாற்றுங்கால்This is clearly a repudiation of Brahminism and its 4-fold varna system and the Brahmins are at the top.
நீதி வழுவா நெறிமுறையின் - மேதினியில்
இட்டார் பெரியோர், இடாதார் இழிகுலத்தோர்
பட்டாங்கில் உள்ள படி.
(There is nothing other than two jAthis, considered opinion says living an upright life, those who are generous are great and those who are greedy are low, that is what great texts say.)
The Bhakti movement was in its zenith around this time. At least the SV Bhakti literature is full of advice to those who considered themselves as high-born. One SV acharya was quite graphic, he says the scholarship of anyone who takes cognizance of a bhaktha's caste is like ornaments upon a dead body.
Coming little close to about the 14th century, there is a poem called Kapilar Agaval.
பார்ப்பன மாந்தர்காள் பகர்வது கேண்மின்Is this not an open challenge to Brahminism?
ஓட்டிய மிலேச்சரூணர் சிங்களர்
இட்டிடைச் சோனகர் யவனர் சீனத்தர்
பற்பலர் நாட்டிலும் பார்ப்பா ரில்லையால்
முற்படைப் பதனில் வேறாகிய முறைமைபோல்
மேல்வகை கீழ்வகை விளங்குவ தொழுக்கால்
(Oh Brahmins, listen to what I am saying
there are many nations in the world, like Ottiyar, Melechar, Unar, Singalar, Sonakar, Yavanar, Chinese; there are no Brahmins in those lands, are they any worse off because of that? As the original matter was separated during creation, you established this four-fold jAthi, but in those other nations the high and the low are determined only by conduct (not by birth).)
An unbiased analysis of available textual evidence will show that the Brahminical varna/jati system always faced opposition from at least a section of the learned class.