Proto-Indo-European (PIE) was never a single language. Let's all repeat that over and over again to ourselves. Depending on one's level of knowledge of linguistics, one might either be shocked by this newsflash or one might be annoyed with being retold what you think you know about PIE. Yet, as I talk with others online, I'm reminded that many, even when seemingly well read up on linguistics in other respects, often still have failed to understand the fullest implications of the Wave Model of language change which informs us, among other things, that languages don't evolve in a simple, straight line.
If "language" is nothing more than a "package of linguistic features" and if each of these individual features can spread in their own ways across a geographical area and amongst a community of speakers over time, then not only, despite the usefulness of comparative linguistics and reconstruction, can there never have been a single Proto-Indo-European language in the strictest sense but that these Proto-Indo-European features we reconstruct have inevitably emerged from many divergent locales and times within the obscure protoplasm of a more distant *Pre*-PIE dialectology. The insight of languages being nothing more than packages of innovation waves is as vital to modern linguistics as wave-particle duality is to modern physics.
Clear as mud? Let's carry on and maybe what I'm getting at will eventually make more sense for everyone. We could use Middle English as an example of how the reconstruction of a language isn't as simple as tree diagrams make it out to be.
If someone asks, What is the 3rd person feminine pronoun in Middle English?, one may respond with all of sche, scho and heo. However, even if one only responded with one of these answers, one would still be correct even if still failing to be comprehensive. A single answer here would be most correct for a single region of Middle English such that sche is a northern variant, scho is from east midlands speech, and heo is from southernmost regions. Looking through time from the Middle Ages to Modern English, we sit like oracles in front of our computer screens with the foreknowledge that eventually she will have displaced or at least marginalized all other variants of this pronoun that were existent in the previous stage of Middle English regardless of their intriguing, respective origins.
What have we learned from this? Again, that language doesn't move in a straight line, of course. So why do we keep thinking that PIE or other theorized protolanguages must?
So back to PIE, I've shared openly before in Paleoglot: The trouble with the PIE 1st & 2nd person plural endings (3) my detailed reasons for why I think that the development of 1pp and 2pp suffixes *-mén(i) & *-tén(i) (as suggested by Anatolian and Greek dialects) must precede the development of *-més and *-té (as found in other PIE dialects). However, in none of this have I intended to impose a single set of plural pronominal suffixes on PIE itself. I urge those interested in theories on Pre-IE made by me, or for that matter by any others, to understand them in the context of an eternal dialectal pluralism. Imagining evolving dialect maps across eons in your head, even the surface notion of it, may be exceedingly more complex a thought than one is used to, but it also gives us a more realistic picture of how languages and protolanguages have evolved. That way, when we ask, What were the 1pp & 2pp active primary pronominal endings in Proto-IE?, we may learn to say, in fashion parallel to the Middle English example above, all of *-méni & *-téni (Anatolian); *-mén & *-tén (dialectal Hellenic); and *-més & *-té. We will also learn to look past the mere illusion of self-contradiction in this multivalued response supplied.
 Hogg/Blake, The Cambridge History of the English Language: 1066-1476 (1992), p.119.