The Indo-European religion is very adaptable for sure.
HOWEVER, Hinduism as we know it emerged much later, in the Medieval era. They didn't merge native religions, so much as they came to encompass the local cults (which range from Vedic religions to unrelated cults). Ancient India was more like China in that regard, and the development of Hinduism was like the development of Shenism (the Chinese Folk Religion), and it's specific philosophical schools of thoughts akin to that of different Chinese religious schools and sects.
Buddhism is older than Hinduism in this sense— and Buddhism was not a reaction to the Vedic faith (although the Buddha frequently engage Brahmins in debates) as the Shakya did not have Brahmanism as a major influence in their culture. Buddhism co-opted the Vedic cosmology, but rejected almost wholesale the entire faith system.
Agree with the first part- disagree with the second.
It is definitely true that the Śramanic traditions that arose were of a different thought system to the more theistic Vedic fold and represent the re-emergence of the myths and cosmology of a pre-Aryan upperclass. However it had been co-opted into the Vedic fold to an extant as Upaniśads and the Atharvaveda mention a lot about the ‘sky-clad’ Keśin monks who ponder the meaning of reality and the divine.
The region east of Kāuśambi most likely lay outside the fold of what is recognised as the organised Brahmanical fold of the Kuru-Pañcalas but were still Indo-Aryans that followed a faith that was at it’s core Indo-European in heritage and most likely worshipped and revered a similar set of deities. The Atharvaveda even mentions the Magadhas, labelling them as impure and barbaric but takes painstaking care to not call them ánārya or non Aryans, a label that it does offer to the hill-tribes that lay north of the Kuru country.