I'll try to structure some thoughts at the moment:
1. 10 KYA, the surviving people from the final Upper Paleolthic sites follower to glacier up to the Ancilus (Baltic) Sea and the Upper Volga.
2. Mesolithic burials and cultures are known along the lines of Veret'e, Oleni Ostrov, Zveyneyki, Butovo and later Elshanka. Probably this can be taken as the reference point of proto-proto-IE. This is consistent with the common northern vocabulary of the flora and fauna of the primordial PIE vocabulary.
3. Indo-Europeans from Khvalynsk were the first to tame a horse,and spreading this innovation to all neighboring Indo-European cultures. They themselves were probably the ancestors of the eastern yamnaya, and therefore no modern Indo-European people, who comes from them. Probably it is an extinct branch, and also dissolved in other nations.
4. Almost all modern Indo-European populations, including Indo-Iranians, come from more Western cultures. Such as the Middle-Dniepr and others.