In fact, there's technically nothing that could ever prove Satoshi's identity. Signing from early addresses or Satoshi's PGP key would only muddy the waters. Private keys can be compromised due to security mistakes or even encryption vulnerabilities.
You are right, but by that logic you can't actually prove anyone is anyone online. I can't prove I'm the same person who made that last post, and theymos can't prove he is the same person who opened his account. If someone showed up with signed messages from Satoshi linked addresses and his PGP key, with detailed knowledge of bitcoin, was able to display knowledge of things like the content of Satoshi's PMs on this forum, was able to convince theymos he was genuine and have his account unlocked, then I suspect the vast majority would see that as sufficient proof. Certainly the media would, and anything he said or did would be reported as "Satoshi says..."Quite right. Posessing some keys (of a car, house, email account - bitcoin wallet) ,doesn't prove real ownership - not on showing, using, whatever.
Real 'evidence' needs a lot of more - esp in real world, like receipts of registering all the web sites, ppl who have Counter checked the White Paper,... telling things not known before in a decent way.
Evidence is a tricky thing out of the math realm - even for judges
