Not sure Muriel's, "Git outta mi town" letter is any legal legs.
Thought the District of Columbia is under the purview of the federal government
The District used to be under total congressional control. Then Congress opted for home rule and local elections. In the event of a crisis, the federal government can take over the district and rule it, but that opens up a can of worms.
The mayor's pronunciations may not have any legal validity, but they are intended to cast blame for destruction of the District on Trump when it happens.