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 key*They 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?
*(1cdced0646879da917d4d9eba9669329c8feb29425d5b192f722a00eebc0d216, a4969ce2061614087ba4a631618f322cbda3dddaf79205db95bba4d24939701e) - (876ca85b08d1adb23639410084ea4953cfda40c676f8e10be5dd08eefee50655, 27ff2580ee182aa46f1bde2fa3a3c80bde963e7fba24a1e60f4d81d1cf9a03ea) = (da2ab1d2001405f6777c44a2f42abdd454957db48a418acaf97ae7aa01666b59, c1c6badb50fae1358cc593581d645f1fe15f202dc9a1a6840d491e86fd43dccb)