But then ... we can be sure that whoever created that list, has the private keys of the first 2 addresses too = has all private keys
No. Simplified equation again: (Bold parts are private keys that are owned by them, or can be calculated by them)
1C7X4UWpSa4GteWHaRBm49fMCC2SNvJQF's public key + 1W7PDetXCcAbXnN6YQyWmAdz65WZecJs5's public key + 1r7VRs5hwFNaqWSMdAGZVoQ7uQhsesRqG's public key + 1i7JYfJiXf5ARAysJaRaECLLcnrx1Gcuw's public key + 1g7nBFZkyET8TPXBoxzBYA83XPJzwDCVT's public key + 1h7djfQ2MjojsRJQdvn6jNuJZZB9oFYLm's public key + 1t7MqxnqwmwooDjKnvV9AFkiktqUvvxkq's public key = 17mZRodKy5ufNqJVsyKg1bEt81AnRkkh9L's public key==>
1C7X4UWpSa4GteWHaRBm49fMCC2SNvJQF's public key + 1W7PDetXCcAbXnN6YQyWmAdz65WZecJs5's public key + 1r7VRs5hwFNaqWSMdAGZVoQ7uQhsesRqG's public key + 1i7JYfJiXf5ARAysJaRaECLLcnrx1Gcuw's public key + 1g7nBFZkyET8TPXBoxzBYA83XPJzwDCVT's public key + 1h7djfQ2MjojsRJQdvn6jNuJZZB9oFYLm's public key = 17mZRodKy5ufNqJVsyKg1bEt81AnRkkh9L's public key - 1t7MqxnqwmwooDjKnvV9AFkiktqUvvxkq's public key==>
1C7X4UWpSa4GteWHaRBm49fMCC2SNvJQF's public key + 1W7PDetXCcAbXnN6YQyWmAdz65WZecJs5's public key + 1r7VRs5hwFNaqWSMdAGZVoQ7uQhsesRqG's public key + 1i7JYfJiXf5ARAysJaRaECLLcnrx1Gcuw's public key + 1g7nBFZkyET8TPXBoxzBYA83XPJzwDCVT's public key + 1h7djfQ2MjojsRJQdvn6jNuJZZB9oFYLm's public key = 1LSpmoBBWydsTjanZK9Lk1j43PnSPhRRmL's public keyThey can only prove they own all the addresses listed in OP (or just those with the 50 BTC), if they sign a message from every address except 1. They might as well own 1r, 1i, 1g and 1h; if they don't provide a proof from either 1C or 1W, they don't own anything.
But, let's say they did. Yes. Let's take this hypothetical scenario wherein these fat, loser, no-life scumbags with no ethics did provide a signed message from these old 1C & 1W addresses, for the sake of Craig's majesty. So what? Does that provide a proof of anything other than they own some bitcoin?