They never have to give an interview. They are suspects in a crime. If they refuse to give a non-criminal IA interview, they can of course be fired immediately under Garrity rules.
The department can require an interview for internal use to see if department policies were used, if training or procedures failed, etc. THAT interview is legally “coerced” because it is under threat of termination if they do not cooperate.
It cannot be used by prosecutors because the statement was given under duress.
As for the criminal case, no statement is ever required there.
SO my guess is this guy will never give an interview. The lawsuit will proceed. And a criminal case will go to a grand jury to see if they think it was an accident of poor training and selection, or if they want to hit the Somali with a manslaughter charge.
But none of this will happen in 137 days is my guess.