And as far as Biden being portrayed by Hur as a "kindly, well-meaning, forgetful old man", that should play no part in actual guilt or innocence (although it might still influence whether a prosecutor brings charges) correct?

I mean, a little of both?

Again, the crime is deliberately refusing to give back national defense information when the government asks for it. If Biden genuinely didn't know that the Afghanistan documents were in his possession, he wouldn't be guilty. This particular statute is a specific intent crime; in order to be guilty, the person would have had to be aware that they were in possession of the covered information and knowingly deciding not to give them back.

Clearly Hur felt that Biden could convince a jury that even if the Afghanistan documents were in his Virginia house in 2017, and even if he had seen them there, that he wouldn't have remembered that those specific documents were in his possession when he was asked to return all classified documents. But there's always the possibility that that's true - that he genuinely forgot that those specific documents were in his possession, even if at one point he had seen them there.