Mueller is running into a problem. He wants to convict and sentence in near secret, and that only works as long as everyone plays along. When people stop playing along and fight back, Mueller has to follow the rules like other prosecutors which makes him accountable. Given his history of bungling sensational cases, Mueller really wants to avoid explaining his actions on the record in front of a judge and jury. Trials work both ways.
Why doesn't Mueller cancel the plea deal?
I suspect it's because a trial would be a disaster for him.