He can't not testify because then HE will lose his plea deal and get the DP put back on him. His plea deal was to get the DP dropped because there was too much evidence against him, his trial was 4 months away, and his attorneys likely leveled with him that killing 8 people would most likely get him on death row.
So in exchange for dropping the DP he had to do 3 things. Plead guilty, give a proffer which means testimony about the crimes, then testify against any of his family members who go to trial.
If Billy decides not to go to trial and pleads guilty, then soon Jake can get a sentencing date set. Same for Angela. But of course if Billy goes to trial it will be a long time before Jake and Angela get sentenced. They will sit for months in jail waiting for their day to testify in Billy's trial.
George is going to be the first Wagner to be sent to prison. The day he is sentenced he will be whisked away to an Ohio prison intake reception center.
drc.ohio.gov
Prison Intake Inmates sentenced to the Ohio Department of Rehabilitation and Correction are processed through one of three reception centers, based on their gender and/or county of commitment.
Reception Summary - Ohio
Prison Intake information.
Processing Summary.
Processing Breakdown for day 1, day 2 and day 3.