I'm not sure if the steps you propose here would work in practice. Even if they would, it's all too complicated for my taste.
most buyers wouldn't want to bother with all these requirements, even if they're legit. passport, selfie and letter of authorization just to buy a little BTC with paypal? no thanks! i would never do that!
I would rather look for some alternative method of sending cash that has no chargeback possibility. And do these intermediate steps, like selfie, ID, letter of authorization really prevent chargebacks?
Also, there is no escrow in this scheme, so it looks to me that the seller is not protected after sending the funds to the buyer.
they don't prevent chargebacks, but they would probably deter a lot of buyers from trying. i suppose the letter of authorization would bolster the seller's case in a dispute, but who knows how paypal would decide?