Discard any mail in ballots counted without challengers/ observers present. That is the correct remedy and only remedy. They can also throw out the other illegal votes if they can find them. I don’t think they can because nothing identifiable. They could also just throw out the entire counties totals where the illegal activity took place.
Zeroing in on all ballots counted after the stop counting order came through -an obviously coordinated thus interstate issue- identifies those ballots as part of an interstate crime. To contest state by state their illicit obstruction of observers from seeing specific ballots being counted is a state by state issue not an interstate issue (RIO anyone?).