The servo design can't be expected to accommodate a manufacturing defect, like what happened in the Copterline accident. The pressure filter is there to stop particles from the hydraulic pump getting to the servo should the pump fail. Gross contamination of the servo due to the servo itself failing isn't a design point, in terms of fail-safe, past what the bypass spool is supposed to do. The only way to counter an un-commanded extension/retraction of a servo is to depressurization the system at the servo inlet, which is the purpose of the servo shutoff valve.
As far as I know, in the Copterline accident the crew didn't switch off the servo when its power piston went to full extension. I don't think they knew they were faced with a hydraulic problem. In any event, switching off the servo might not have saved them because both servos had a problem, and as we all know, you can't switch off both servo shutoff valves at the same time (which is a design point). Again, as far as I know, if the after start hydraulic system check had been done properly then the Copterline crew should have realized there was a problem with the servo. But that was a different accident. I doubt that anything relevant to that accident, in terms of flight control system problems, can be applied to either Bristow accident.