Namaste
The OP started off with a list of "bullet points" regarding what Hindus share (perhaps this can be called a list of "Hindu-ness") that makes us Hindus even within our very diverse family.
Others volunteered more ideas to this list. Then the entire thread became sort of a bing and bang over "AIT yes", "AIT no", as well as a small minority that appears to think Hindu is some sort of race and not a practice or adherence or revelation of Dharma.
I tried to add a couple of items to the "list" regarding temples and goshalas, it didn't revive the "list" at all, it must have been so stupid (accept my apologies) that instead the bing and bang on AIT continues.
So instead, I have now decided to heck with the "list", I am joining in on the AIT fun.
So the first thing I did last night was to try and encapsulate some of the popular theories to see if any make sense to me. Of course the problem, the summary (below) is dreadfully lacking since proponents of one or other of these theories argue even with themselves over the details of their shared invasion or no invasion, so my summary is probably lacking and controversially in doubt since no one can prove any of this.
Here is my summary (dreadfully lacking), of which I do not side with any of them but slightly agree with one which I will clarify at the end:
-----------
1) Indo-Europeans from parts of greater-Russia and Caucasus Mountains riding chariots invaded the subcontinent of what is known today as India 3,500 years ago, shaping the physical and religious landscape of the region and bringing Vedic gods and religion that was over time assimilated with local, native goddess religion and village deities that reflected yet even more ancient incursions not yet fully understood. The word "Aryan" was improvised into the theory in a joiint effort by British as well as Brahmins who hated Sudras and Dalits considered dirty dark skinned half-human slaves and potential competitors over water, land and mineral rights of the Indo-Europeans. Some of the elite lighter skinned aristocracy then started to mate with the slaves whose offspring became various jatis or castes. This is the classic invasion-subjugation-assimilation theory.
2) Dalits were the original Indians who were largely exterminated in many regions of the Indian subcontinent by nomadic Caucasoid-Turkish-Hittite people of the steppes riding on rather midget horses in a series of 3 invasions between 5,000 and 2,500 years ago. Dalits are related to Austroloids who lived in very small populations and are distant relatives to pygmy type peoples who have been in the subcontinent and nearby islands for 30,000 years or more. The first invasion entered Northern India, but the invaders essentially became "pooped out" with exhaustion as the invaders progressed ever further Southeast due to distractions of yet a later invasion from like "Aryans" thousands of years after the first invasion which brought yet later variations of the gods and a new "quarter horse" cavalry using larger horses that could nevertheless turn at sharp corners while mounted warriors could shoot arrows while riding. This resulted in warring-states between Indo-Europeans in the region that included the use of elephants, and at a time when Dalits also started to learn the military methods of the Indo-European "Aryans". By the time of yet a third Indo-European invasion 2,500 years ago, the Indo-Europeans had largely started to abandoned war in place of settled agricultural life to feed the growing population. This is classic Never Ending War-Later Sick of Fighting and I just want to eat Rice scenario.
1 and 2 footnote) Both 1 and 2 invaders like rivers.
3) There never was an invasion of Bharat or India, which has been the same approximate size of Pakistan-India since time immemorial. Except of course the Afghani Muslim invasions starting in the 1100s AD and the later European incursions and hegemony starting in the 1500s AD by the British, French and Portugese, but certainly never were there other invasions such as the false AIT secret plot and so-called foreign "Aryans", India was always pure of such mleechas until modern times, AIT is simply a conspiracy by the British who were jealous of the ancient glories of Indians and wanted to claim such brilliance as their own by claiming "Aryans" and Europeans are the same, while at the same time hating and trashing all Indians and wanted to steal the gems in the eyes of Hindu "idols" so they could decorate the European crowns with jewels and other such AIT agenda and plots. This is classic Jealously-Conspiracy-Glory-Victimization scenario.
4) The reason there are lighter skinned Indians in the Northwest and darker toned Indians in the Southeast has less to do with invasions, even though many ongojng invasions have been occuring in this part of the world for ages, and more to do with like evolution of skin pigmentation associated with climate found in many parts all over the world. Snce the time of an initial influx of people from the area of Mesopotamia (e.g. modern Iraq and Syria) 17,000 years ago which largely exterminated the Austroloids and pigmies who lived in small populations in India and some nearby islands, over the 17,000 year period those who lived in the Northern regions evolved to lighter skin favored in colder climates for survival, while those who lived in the Southern regions evolved to darker pigment favored in warmer climates for survival. Dalits have no relation to the original pygmy peoples but were ostracised due to darker skin tone by lighter skinned clans. If Indians from the South were to have migrated to the North such as to Kashmir, they would eventually evolve to lighter skin, while Indians from the North if they would migrate to the hotter regions of India and to the South they would evolve to darker skin and look more like South Indians. There were all sorts of invasions as well from Western areas outside of the subcontinent, from Eastern areas of Asia, from the North including from areas known as Russia over the next thousands of years, but there were outward migrations as well to other areas of the world and as far away as to Northern Japan, but there was a period of time when the region known today as India was isolated due to a pandemic of disease about 4500 years ago, likely bubonic plague, which isolated India for a period of 500 years from the rest of the world. None of this has anything to do with Vedic culture or Hinduism which spread out in the reverse direction from India to Mesopotamia and beyond, though it took on localized variants in those regions. This is the classic Everything is based on climate and disease scenario.
5) Alien beings from other planets landed in India and other parts of the world and showed them "technology" and "secrets" and bred with humans. These beings were worshipped as gods. These beings were likely lost or fleeing from their outerspace homelands, eventually they left to an unknown fate but more aliens may come in the future. This is classic just look up scenario.
6) Bhu Devi is a "Hindu". She is all of Earth Devi, and not just India. She is the consort of Varaha, because Bhumi Mother Earth was saved by Varaha because Earth is special so She married Varaha. All of Earth including the animals and birds are citizens of Bhumi and Bhu is their mother as well as Gomata. Hinduism can be anywhere on the Earth since all of Earth is Bhu and Bhu is Hindu Goddess Herself. The Hindu Dharma is on other Lokas as well and not just Bhumi. Bharat is a name for one of the most glorious of Dharma protecting Kingdoms. It was much larger tban what is thought of as India today, at least 4 times larger and in the times when the words of Vedas were put to written words, the continents and sea levels and star positions were different than today. Some parts of what was Bharat is now under the oceans, some other parts of the world have now arisen from the waters. Some islands have crashed into other continents. Peoples have been divided and rejoined as the land has shifted and winds have blown them here and there. This will all come to an end, but will return again.
-----------
I subscribe in part to number 6. But have some issues with it as well.
Om Namah Sivaya