Vedic Mitra was invoked along with other Vedic gods as far away as current day Syria/Turkey
[..] Indic Mitra is first attested in a 14th century BCE Mitanni inscription in which an Indo-Aryan king of Mitanni invokes the gods Mitra, Indra, Varuna, and the Nasatyas as guarantors of his sworn obligations.[..]
Also in the treaties signed by Mittanis and the Hittites.
Mitra always appears with Varuna. In the beginning of Rig Veda, Varuna is the king of the Vedic gods and is resplendent and benevolent..but towards the end, he loses his luster and becomes a minor god..mean, dull and almost cruel as Indra becomes the new head after slaying Ahi , the serpent demon who captured Apas, the water goddess(apam-napat or apa also appears as a Zoroastrian deity)
One theory is that this event of Indra rescuing apa from ahi by releasing her frozen waters(ahi froze the waters and indira used his lightning bolt/thunderbolt weapon vajra to melt the ice so she will flow down back from the frozen state) can be times to one of the ice ages ..and younger dryas event + the sedimentation layer might have been after that..which would have been due to thehastened melting of the ice when the comets burned down..causing the ‘great flooding’..an event that is recorded in every world mythology.
Back to Ahura Mazda/Varuna split.. there were two priests classes..brighus and angiras..angiras took some books and went east..the brighus kept some of the Bhargava texts. Even today, the Vedas are bit complete and many parts of it are considered ‘lost’
Shri Jatindra Mohan Chatterji argues that Zend Avesta is the Bhargava Veda text. He asserts that the Bhargava Veda the missing Book of the Bhrigu Angirasa Samhita (The Hymns of Atharvan Zarathustra – Published by The Parsi Zoroastrian Association, Calcutta, 1967).
The Indo-Iranians likely became divided because of differences in opinion about methods of worship and accent on certain principles. Likely leading to the split of the original Aryan land into India and Iran..east of river Sindhu became the land of Hindus.