I think "once the DOT makes its decision" really means "once the DOT comes up with a way to justify the decision they already made".
Rather than require UA to re-instate the tickets, the DOT could just fine UA some dollar amount and close the case.
DOT cannot require UA to re-instate the tickets. No authority to do so.
IMO, I think this is basically over. I am not sure there will even be a fine. I know nothing has been decided, but, as the weeks go on, I think this will just fade away.