i think yes, i'm not an expert but AFAIK if they don't know the deposit address then how can they remember the receiving address plus you asked if authorities have the hacker address (containing $300 million), they i think it does not matter whether mixer knows the deposit address or not because they already have the hacker account details and can detect whenever the hacker try to make deposit on Mixer.
I don't think so, because they might not co-operate with them you can take the example of latest ban on Coinmixer, did they co-operated? idk, But in my view if such illegal cases happens then at least these mixer should co-operate with them because at one side we talk about it's mass adoption and other side they don't want to co-operate with authorities, i think they should co-operate with them in vital condition like these and should take extra measures to keep other anonymous while exposing only the hacker account.
I don't think they can block addresses rather other than using blocklist for addresses, answer has many probabilities as if they co-operate with authorities then authorities will ask mixer to not block it in fact they will ask them to seize it idk if they can access the seized money, but by blocking it they will lose the chance to get access to stolen money back. And If they are not co-operating with authorities then they could block it because why would they need problems. Honesty is the best policy.