@gulliBell
The pressure filter is there to stop particles from the hydraulic pump getting to the servo should the pump fail.
Let's be precise. Between the Servo Actuator pistons and the spool valve there is no any filter. All the particles and debris what are removing from the pistons or piston rings will have a direct routing to the C3 return ports. The design issue (the design point) is the spool valve's C3 return (fluid) ports and the Servo Actuator structure. If the return port blockage occurs the helicopter will lose its controllability immediately.
gulliBell, this is the first issue what I can fully agree with you. The only way to get the helicopter to be flyable again (if the helicopter will come immediately unflyable) is to switch or #1 hydraulic OFF. This will release (expected) affected #1 Servo channel off (depressurized). If switching off #1 doesn't help then the next step is select the #2 hydraulic OFF. This will release (expected) affected #2 Servo channel off (depressurized). The most likely the second attempt will release affected servo channel and the helicopter will be a steerable and a flyable helicopter again.
The major challenge is: due to the servo system design the helicopter caution system won't make any cautions to the crew. The crew can't identify what is the failure. There is no warning or any cautions. The RFM doesn't have any emergency procedures how the crew should proceed if this emergency scenario will happen.
I doubt that anything relevant to that accident, in terms of flight control system problems, can be applied to either Bristow accident.
For sure, its too early to me or you to make any conclusions what has happen or what hasn't happen (these two Bristow accidents).
It's nice to see that you have a good understanding of the hydraulics and the flight control systems. Sorry, I made earlier a wrong conclusion of your capability and your knowledge due to your FCR/CVR comment which was little bit confusing as your earlier posts #521, #523 and #529.