There could be a legally binding contract stating that any deletion request must be forwarded to all parties it was send to, and that upon receiving such a request the data must be deleted.
I do not think this would be unreasonable to ask to servers, especially as this deletion receipt could be fully automated.
Maybe, but consider that federated servers may be located in entirely different legal jurisdictions, so this might be hard to create, let alone enforce.
There could be a legally binding contract stating that any deletion request must be forwarded to all parties it was send to, and that upon receiving such a request the data must be deleted. I do not think this would be unreasonable to ask to servers, especially as this deletion receipt could be fully automated.
Or there could be a delay of one minute before posts get federated, giving the user the option to quickly delete a comment or post.
Maybe, but consider that federated servers may be located in entirely different legal jurisdictions, so this might be hard to create, let alone enforce.
When writing a contract you can just specify which legal system the parties agree to use - this is quite common.