1) As this benign attacker accepts any new block by other miners, is it again illegal to have more than 50% of the total hash-rate?
It is not illegal to have any percent of hash rate, you are allowed to have it. If you meant whether it breaks the bitcoin protocol, then the answer would be no if they are always benign. However, once they do turn malicious, they can harm the network. Refer to this link for what a 51% attacker can/cannot do.
2) If yes, who can detect such a benign attacker?
I will present two arguments, one in a complete theoretical world and another one that uses some information that is not available to the bitcoin protocol.
You cannot detect them. There could right now be a benign attacker which you describe and we cannot know about them. For a more concrete argument, consider the following two scenarios.
Scenario A: In this scenario, all the miners are honest and create certain blocks and honestly extend each other.
Scenario B: 60% of the miners are malicious(benignly as you describe) but choose to act as if they are honest.
All the block creation and exchanges which take place in scenario A and scenario B are identical. So from the point of view of observer, those are identical. And hence, in theory, we cannot detect such a benign attacker.
However, in practice, I believe that it is unlikely such a benign attacker exists as it would require lots of resources to be owned by a single party.