The first Indo-European speakers were the Hittites. They entered Anatolia and conquered the non-IE speaking people.
I am beginning to believe that after the Hittites defeated the Hatti and Kaska and other peoples belonging to the Hurrian and Mitanni kingdoms,these people were uprooted and forced into Iran. The lost of Anatolia to the Hittites, probably forced
these people to become nomads.
In Iran they probably formed a significant portion of the Proto-Aryapopulation. Here they may have met Indo-Iranian speaking people,who may have practiced a hunter-gatherer existence, that adopted aspects of their culture , especially the religion and use of Mitanni religious terms and chariot culture.
Joining forces with the Mitannian-Hurrian exiles they probably attacked Dravidian and Austronesian speaking people who probably lived in walled cities. The Austronesian and Dravidian people probably came in intimate contact during the Xia and Shang periods of China.
I have to reject the Afghanistan origin for the Indo-Iranian speaking people because the cultures there in ancient times show no affinity to Indo-European civilization.
Given the Austronesian and Dravidian elements in Sanskrit and etc.,I would have to date the expansion of the Indo-Aryan people sometime after 800 BC, across Iran,India down into Afghanistan, since the Austronesia people probably did not begin to enter India until after the fall of the Anyang Shang Dynasty sometime after 1000 BC. This would explain why you declare that "the Vedic and Avestan mantras are not carbon copies of each other", they may have had a similar genesis, but they were nativised by different groups of Indic and Iranian speakers after the settlement of nomadic Hurrian and Mitanni people in Iran.