Actually, I think the OP did say who Krishna is to him.
But I thank you for the history lesson. I find it fascinating, but I believe there are many more discoveries to be made, as India is so ancient and complex. Recent scholarship places the events recorded in the Mahabharata as occurring 5000 years ago, one piece of evidence for that is the alignment of stars mentioned which modern astronomy can prove would not occur in the sky 3000 years ago, but did appear 5000 years ago, so perhaps Krishna actually predates the Aryan migration into India. Hindu scriptures such as the Vedas and Upanishads that we have today were composed more recently than that, as you noted, but certainly there was a long and ancient oral tradition before anything was written down. I would argue that the Book of Genesis is like that as well, mostly oral tradition that was passed down for generations, but still it is the word of God.
If you want to understand who Krishna is, in my opinion, you can look at the reverence shown by His devotees, how He is portrayed in scripture, including how He described Himself. In other words, the same measure Christians have with Jesus, or Baha’is have with Baha’u’llah,