All A did was hack it. Hacking =/= stealing.
You can brute force a key? Bitcoin is done.
Bitcoin being done, nothing of value has been stolen
Read: hypothetical situation.
Hacking is one thing, but publishing your results from your hacks is another, which might be illegal in some places.
Also, if Bitcoin is hypothetically hacked, then it's hypothetically done. I don't see the problem there.
Except here it was not hypothetically hacked here. It's just that the original/rightful owner of those 10BTC used the private key 17 instead of a safe random key.
In a real life problem similar to this one people wouldn't just distribute a private key or create, sign or broadcast a random transaction just because. They would do that to steal those coins and that would make them thieves. So OP should give us the hypothetical intentions of every user here to receive a proper answer.
Regardless I'd consider user C to be a thief if he doesn't return the coins after being told what happened and if the original owner proves that with a signed message or by other means. The other users would be thieves if they did all that with the intention to steal the coins.
Regarding this, it's not the same case. First you can't compare dust/1sat with real value like 10BTC. Also in this case the original owner was unaware of his funds being sent to user C. In your 1sat example it's clearly intentional because there are several transactions.
If someone receives 10BTC once against the will of the original owner then the only right thing to do is to return the coins (minus TX fees). If someone receives spam several times then of course he doesn't have to return it, he couldn't even cover the fees if he wanted to refund the coins.