But while it is clear to us, is there any legitimate way of proving which Bitcoin is still on hold in the addresses owned by Satoshi?
Lopp's blogpost isn't really about Satoshi's coins, but about all coins which are "lost" or are still sitting on addresses belonging to scripts like P2PK where the public key is exposed in a block (normally the pubkey is only exposed after you send the coins anywhere, that's why addresses shouldn't be reused). Thus it would not be necessary to determine Satoshi's addresses.Satoshi is here very likely only namedropped to drive attention to the thread - i.e. as a clickbait
in a forum that's completely legit I think. The original blog post title is "Against Allowing Quantum Recovery of Bitcoin". However, Lopp has used the word "Satoshi's stash" in a video called "Safeguarding Satoshi's stash".What if Satoshi simply did not care enough by now to move his Bitcoin and by the time he wants to move them, they are impossible to move?
Well if such a hard deadline is set and the period to move is long enough I guess Satoshi and all others sitting on Bitcoins on old hard disks in the attic would move. As described above, Lopp proposes a generous migration time.Is there an actual accurate number of how much Bitcoin Satoshi owns in the first place?
There is a quite fundamented estimation by Sergio Demian Lerner, who discovered the so called "Patoshi pattern". See The Well-deserved fortune of Satoshi Nakamoto and this follow-up. We're talking about 980k to 1.1 million BTC (around 4-5% of the circulating supply). Later Lerner made some other posts going more into detail.