Bob-
No matter the type of motor retention, a full blow-by in a sealed booster chamber would make the motor kick.
Suppose the chamber pressurized to half pressure:
300 psi.
Considering the diameter, that is over 1000 pounds force of required retention force. Even the PML threaded aluminum system would fail at the bond joint to the MMT.
I'm afraid even the 304 stainless anchored with 7075 retention ring (the actual 54mm retention device) would have failed as well.
So, while I understand your desire to keep the range safe, a motor ejection from blow-by on a sealed CF rocket is inevitable. This is why we switched to plugged forward closures as well...
So no, I'm not willing to so easily put aside a 1600 man-hour fabrication because of inherent dangers of rocketry. Rocketry is an inherently challenging activity that requires some caution, yes, but eliminating all the risks is impossible and getting on my case about this is really starting to grind my gears.
Furthermore, the ROC RSO approved the rocket for flight after several in-depth inspections. We were placed on a pad fairly far away.
By nature of this activity, things will unfortunately happen. Anyone who says any differently either has figured out a way of cheating probability or is not realistic.