well two big flaws in my eyes pop up is why is dan part of the conversation? he's not related to hasirama anyways. and it was said that kabuto was found on a battlefield surrounded by corpses.
As well as other revelations in the manga that oro freed him from sasoris mind control, and oro didn't work with until the chuunin arc we can assume that sasori placed him on the battlefield under his mind control, hence the "been used as a spy since a small child" and once oro freed him and he got to think on his, own he followed him, That explains him up till now.
But whose kid he is is a mystery but i doubt he's from the leaf village since sasori had control of him (probably from the sand country or further) till oro found him. And if he WAS from the leaf village they'd have a record of his birth at least. and tsunade was never prgnant? sinc ehe's only like twenty the ages don't match up well.
"Knowing is not enough, we must apply.
Willing is not enough, we must do."
- Bruce lee